There have been three flavors of Nikon automatic TTL flash systems, all incompatible with each other:
A) Film TTL, called TTL
B) Early digital TTL, called D-TTL
C) Current digital TTL, called iTTL
So... today, the term TTL has three meanings:
1. TTL is used to generically refer to automatic Through The Lens metering, not necessarily even flash. But if automatically metered flash, then of any type above, generally, without specificity. For example, a menu may say TTL (general automatic flash), but it does what the system can do, which may actually be iTTL (a specific type of general TTL).
2. And also, there are times when TTL can specifically mean the TTL method used on Film cameras, A) above. Simply because film TTL was always called TTL, which made sense back in those times, but it is different now, and we had to find new terms today. The usage context may help decipher meanings. Today, with current Nikon DSLR, it is always iTTL now. But in old gear, TTL may specify film TTL.
3. And also TTL iTTL metering method may be TTL or TTL BL. TTL BL is balanced flash, reducing flash level to merge it with bright ambient.
This can be confusing, but the film system was named TTL, and today's digital flash system is named iTTL.
For example, the cables SC-18, SC-19, SC-26, SC-27 are named "TTL Cables", but that definitely means Film TTL.
The current iTTL system is wireless, and does not use any cable (except a hot shoe extension cable, SC-17, SC-28, SC-29).
A) TTL often specifically means Film TTL, because TTL was also used as the name of the specific automatic flash system used in film cameras since 1980 (except the F6, which can do 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 classics, popular in the late 80's and 90's.
The film camera metered the full working flash power via the flash reflection from the film surface after mirror was up and shutter open, in real time during the actual exposure. When exposure was sufficient, the flash was quenched (stopped). One of the four pins in the camera hot shoe is the quench pin. ( iTTL flash does not use the quench pin, but it still exists, and even for iTTL, it can be used to quench the flash to save power when the shutter closes)
There were two methods to use multiple TTL flash units:
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 control, the individual flashes were not metered, but for TTL, the quench was simultaneous control.
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.
Film surface reflection does not work for digital (anti-aliasing filter), so the D-TTL camera meters a hot shoe preflash reflection from the front of the closed shutter surface (which was painted light gray) after the mirror was raised, but before shutter opens. Hot shoe flash power level was preset from preflash calculations. There was no quenching, and no provision for off camera remote TTL, and no provision for multiple TTL flashes, and no FV Lock. It was a first try for digital flash.
C) iTTL - Beginning in 2003, with D2H and D70 with SB-800, and up until the newest recent current models (This even includes the one F6 film camera). The iTTL flashes are SB-300, SB-400, SB-600, SB-700, SB-800, SB-900, SB-910 and SB-R200 (as of today, 2013). There are also several third party flashes compatible with the Nikon iTTL interface (and a handful 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.
iTTL meters a low power preflash from the flash before mirror is raised, metered using the viewfinder RGB meter. Preflash is with lens wide open, and preflash calculation sets the power level in the flash to be appropriate for the aperture which will be used, with no additional attention or monitoring (no quenching concept as in film TTL). Multiple flash is done wirelessly via Commander, which can control groups of remote flashes. iTTL can individually meter each flash (assuming only one flash in each group), and it sets individual power level accordingly. This adds a delay, but adds wireless flash, multiple TTL flash, and it now also allows FV Lock, to lock a flash exposure for multiple frames, or simply to meter independently of shutter time (FV Lock prevents pictures of subject blinking from timing of the earlier command flashes). More Here.
Both the camera and the flash must have the same capability of course. 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).
SB-600 and SB-800 flashes were unique, in that they were backward compatible with old cameras, and could do TTL, D-TTL, or iTTL, whatever the camera could do. In those manuals, the name TTL in a menu was the general use, but the term TTL in the text descriptions meant film TTL.
The newer models, SB-700 and SB-900 etc., do iTTL only, and are not backward compatible.
A) TTL (film TTL)
There was multiple TTL flash, but there was no individual control of the flashes. All TTL flashes fired at full power, and when the combined light seen was metered as sufficient, all flashes were terminated (quenched) at the same instant. The effect of any close or powerful flash was much stronger than any more distant or less powerful flash.
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 cables), and the most current flashes do not have that three-pin connector. So beware, this "TTL cable" does not mean iTTL.
The 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. The older SC-17 still works the same as the SC-28, except it just does not have the newer shoe pin locking mechanism.
The separate optical slave foot accessory (Nikon SU-4 unit, still available) had an Auto mode, where it quenched the flash when it saw the triggering flash stop (could work at short range). Therefore, this was another multi-flash system for film TTL - it simply followed the controlled hot shoe TTL flash which triggered it. Some current flash models also include this optical slave internally (for manual flash - NOT for use with Commander), still called SU-4 mode in those models menu.
B) D-TTL (early digital)
There is no multiple flash D-TTL capability. Single hot shoe flash only.
C) iTTL (current CLS system)
The iTTL wireless Commander offers the only method for multiple units, and offers true control of individual flash groups (limited to two or three of them), and its default is meter individual flashes and set power of all flashes to be equal intensity at the subject. Then individual compensation can be done. More Here.
We tend to imagine that the term CLS means the Commander, and people often say "CLS" when they actually mean "wireless Commander". But instead, CLS is a communication system between camera and flash, allowing many additional features. The Commander is simply one of the several features supported by the CLS communication.
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 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).
Any system using PC sync cords, optical slave triggers, or any regular radio trigger, is manual flash only (incompatible with iTTL, incompatible with Commander). However, there are a couple of expensive add-on radio trigger systems that do relay the multiple Commander infrared signals via radio, achieving the same TTL Commander system, with radio links.