There have been three flavors of Nikon automatic TTL flash systems, all incompatible with each other:

1. Film TTL, called TTL
2. Early digital TTL, called D-TTL
3. Current digital TTL, called iTTL

But the nomenclature is worse - Today, the term TTL has three different meanings, at least for Nikon:

The context of the TTL term usage has to be mentally analyzed to decipher meanings.

1. TTL is used to generically refer to automatic Through The Lens metering, not necessarily even flash. The menus use it this way, to just mean automatically metered flash. It can be of any type, generally, without specificity. For example, a flash menu may say TTL (general automatic flash), but it means it does what the metering system is going to do, which may actually be iTTL (a specific type of general TTL).
2. The term TTL previously specifically meant the TTL auto-metered method used on Film cameras, A) above. Film TTL was always called TTL, which made sense back in those times, but digital is a new situation, different now, and we had to find new terms today. Today, with current digital DSLR, Nikon's mode is called iTTL now. And there was a short previous first Nikon digital try (until 2003) called D-TTL. But in reference to older film gear, TTL specifically specifies film TTL.
3. In 2003, the SB-800 was the first iTTL flash for the Nikon D2H DSLR camera that could use it. But even the term iTTL has two meanings to consider. It has both TTL or TTL BL modes. TTL BL is balanced flash, which is the default flash metering mode of iTTL cameras. In bright ambient, balanced means it reduces flash level to be fill level to merge it with bright ambient.
   Specifically, TTL BL means balanced, in contrast to regular TTL mode which is regular TTL flash automation, when flash power is adjusted to whatever was actually metered, Not balanced flash, Not reduced by ambient level. The camera menu call it all TTL, meaning whatever the flash is going to do. A few flash models (SB-600, 800, 900) have a menu to select either TTL or TTL BL, but iTTL will be TTL BL unless there was a way to override it. Otherwise, normal bright light meters TTL BL balanced, and dimmer ambient meters TTL.

   Think of iTTL this way. The ambient light is metered by the camera meter, which sets the camera exposure (shutter speed, f/stop and Auto ISO) to a correct exposure. The TTL flash metering sets the flash power level to give a correct exposure with flash. However, those two correct exposures combined is 2x necessary, which is 1 EV overexposed (at least the near part the flash lights is). That was always a problem with film TTL, when the early computer chips used in cameras back then did not have sufficient ability to do more. It worked OK indoors (in dim ambient compared to a sunlight exposure) but bright sun had to be compensated by the user. So then -1 2/3 EV TTL was the automatic standard sunlight flash exposure (which is 24% flash, and probably 1/3 EV overexposure of the near subject). But today, iTTL mode TTL BL balance flash automatically similarly backs off on flash power for the flash to just be fill level (unless a flash menu to set TTL instead of TTL BL specified otherwise). In any case, the sum of any two lights is always brighter than the one brightest, so even though flash power is backed off, there could still be mild overexposure, perhaps 1/3 EV or so, often acceptable.

So what we casually call TTL flash (Through The Lens metering) might correctly refer to film TTL, but is probably instead iTTL today. And in iTTL, it is possibly TTL regular flash mode, but most often is TTL BL balanced mode. We sort of have to know what we are talking about. 😊

This can be confusing, but in short, the first film flash TTL system was named TTL, and today's digital flash TTL system is named iTTL.
For example, the Nikon cables SC-18, SC-19, SC-26, SC-27 are named "TTL Cables", but for those cables, it definitely means Film TTL.

The current iTTL system is wireless, meaning it does not use any cable (except it can use a hot shoe extension cable, Nikon SC-17, SC-28, SC-29). These hot shoe cables work with either film TTL or digital iTTL. Note that Wireless does NOT necessarily mean radio, it just means wireless, including optical communication by light signals (flash pulses in Commander mode). TTL BL requires normally bright ambient, and the iTTL flash can become TTL BL. Dark and underexposed ambient means the iTTL flash is TTL, meaning full exposure of the metered flash, Not TTL BL balanced.

And for flashes and cameras that can provide a PC connector, a standard PC Sync cable can be used for manual flash.

How Nikon Flash Actually Works

A) TTL - We casually say TTL for any automated flash, but TTL today technically might mean Film TTL, because TTL was also used as the name of the specific automatic flash system used in film cameras since 1980 (except the last Nikon film F6 model, which can do both TTL and iTTL too). Nikon's first film TTL flash was the SB-12 for the F3 camera in 1980. The SB-24 and SB-26 were later classics, popular in the late 80's and 90's, before digital.

There were two methods to use multiple TTL flash units (meaning film):
1. The AS-10 adapter and the sync cables with three-pins (SC-18, SC-19, SC-26, SC-27), could quench remote multiple flashes (controlled by the camera via a cable to the flash).
2. the SU-4 optical slave (foot-mounted on each remote flash) had an AUTO mode which followed the triggering flash, quenching when the triggering hot shoe flash stopped (controlled by the TTL flash on the camera — when it stopped, the remotes stopped).

This was not individual flash control, the multiple flashes were all metered simultaneously as one, but the quench to off provided by the three wire cables was simultaneous control for multiple wired flashes (providing overall exposure, but no lighting ratio control). However, wireless optical slaves did not offer quench and were in full Manual control. However, the effect of any close or powerful flash was much stronger than any more distant or less powerful flash, unless there was manual control.

The three-pin cables (Nikon SC-18, SC-19, SC-26, SC-27) were named "TTL cables", specifically meaning Film TTL (only). The three-pin cables provided communication of trigger and quench to all flashes. Current iTTL cameras do not use that system (iTTL does not even use any cable, other than a hot shoe extension cord). And the current flashes do not have that three-pin connector. So beware, this "TTL cable" does not mean iTTL, it means film cameras.

The Nikon SC-28 type of cables (hot shoe extension cords, including SC-28, SC-29, and older SC-17) also have a three-pin socket (for connecting older film TTL flash units). These can also do iTTL or D-TTL with the ONE single flash in their hot shoe, but not multiple flash, except film TTL.

B) D-TTL - early digital cameras, 1999, D1 series and D100 (and D2 family is backwards compatible). Flash models ending in DX were D-TTL flashes: SB-28DX, SB-50DX, SB-80DX. Also the SB-600 and SB-800 were backwards compatible with TTL, D-TTL, or iTTL.

C) iTTL - Beginning in 2003, with D2H and D70 with SB-800, and up through the newest recent current models (This even includes the one F6 film camera). The iTTL flashes are SB-300, SB-400, SB-500, SB-600, SB-700, SB-800, SB-900, SB-910, SB-R200, and SB-5000 (not in that order, the SB-800 was first, and of these, SB-500 was most recent). There are also several third party flashes compatible with the Nikon iTTL interface (and many of those also compatible with the wireless Commander Remote mode). The iTTL metering is done by the camera system, and the flash simply has to accept the proper power level command.

The older SC-17 hot shoe extension cord still works the same as the SC-28, except it just does not have the newer shoe pin locking mechanism. Also the SC-28 places the cord at one side instead of in front (in front on SC-17 can block the internal flash door from opening fully). The SC-19 adds an AF-Assist flash sensor at the hot shoe end, always pointing forward to the focus point (whereas the one in the flash might not be aimed at the focus point if the flash is off-camera).

The iTTL wireless Commander offers the only Nikon method for multiple flash units, and offers true control of individual flash groups (limited to two or three of them). Its default mode is to meter individual flashes and set power of all flashes power level to be equal intensity at the subject. Then individual compensation can be done from there.

We tend to imagine that the term CLS means the Nikon Commander system, and people often say "CLS" when they actually mean AWL for "wireless Commander". But instead, CLS (Creative Lighting System) is a communication system between camera and flash, including many additional features. The Commander system named AWL (Advanced Wireless Lighting) is simply one of the several features supported by the CLS communication system. AWL is the Commander system providing multiple wireless remote iTTL flashes (individually metered and adjusted by the commander and camera).

Flash Naming Nomenclature and Compatibility:

• The first manual Nikon hot shoe flash was the SB-1 in 1969, but the first Nikon hot shoe thyristor model (automated, the flash metering the exposure itself) was the SB-2 model in 1972. Thyristor flash had a photocell in the flash body to meter the light reflected back from the subject, which quenched itself off when exposure was deemed sufficient. Subsequently film TTL metering worked very similarly from a photocell in the camera, metering light reflected from the film surface, which had a big advantage of seeing only what the film and lens saw.
• The Nikon two digit models (like SB-24) were for film cameras, and for film TTL since the SB-12 (in 1980 for the Nikon F3 camera).
• Digital D-TTL began in 1999. The DX models (like SB-50DX) did D-TTL (the same model numbers without the DX were film TTL only). The SB-50DX and SB-80DX worked with either film cameras or with D1 digital bodies (D-TTL). The SB-28DX was D1 digital only.
• iTTL began in 2003. The three digit models are iTTL models. The SB-600 and SB-800 flashes were first, and quite special, because they were backward compatible with old cameras, and could do TTL, D-TTL, or iTTL, whatever the Nikon camera would do. In those user manuals, the name TTL in a menu was the general use, but the term TTL in the text descriptions meant film TTL, and iTTL meant digital. The later models, such as SB-700 and SB-900 etc. do only iTTL, and this automation is not backward compatible with older cameras.

For any automation, both the camera and the flash must have the same capability. Otherwise, the flash only works in manual flash mode (or possibly an Auto non-TTL flash mode, where the flash itself meters and controls the flash).

Older non-iTTL flashes can still do Manual flash mode on the newer iTTL bodies. And also can still do any flash Auto mode (where the flash itself meters the exposure), however, if the old flash cannot do the newer CLS communication, then the camera cannot otherwise communicate with the flash (cannot send ISO or aperture info for example, so it would have to be entered in the flash manually.) But the iTTL body can still trigger the Manual flash mode. And any simple optical slave triggers should still trigger manual mode.

Multiple flash units in the Nikon System

Nikon provides this DSLR and Flash Compatibility Chart. It mentions older iTTL flashes, but does not mention film TTL cameras.

The first Nikon digital models D1 and D2 used the D-TTL system for automatic metered flash, but since then (the last 20 years), the iTTL system is used. HSS flash came out on the Nikon SB-25 flash in 1992, called FP High-Speed Sync then, but it did not have the Auto FP feature yet (automatically shifting modes with shutter speed).

There are no iTTL cables whatsoever. Multiple iTTL is only done wirelessly via the Commander (called Advanced Wireless Lighting, AWL). So (excepting the hot shoe extension cables like Nikon SC-28 for ONE iTTL flash), any cable when used with an iTTL camera can only be used with multiple flashes in Manual flash mode. The iTTL multiple flash method is the wireless Commander (not using any cable).

The word wireless just means no wires. It does not necessarily mean radio, the Nikon Commander system instead uses the infrared light component in the visible flash. Flash is visible light, but which also includes a strong infrared component. The remote flashes (and the SU-800 commander) do have infrared filters to filter out the visible light, which we may call infrared, but this filter is not required, because the visible flash naturally also contains a strong infrared component.