There is a lot of confusion about the whole TTL universe. As far as I understand it (about 2 cm worth) eTTL is Canon's proprietary TTL system that lets Canon cameras talk to Canon strobes. iTTL is Nikon's version of the same. If you are shooting a Canon camera and using a housed Canon strobe then eTTL works fine (if you have the right electrical connections). Same for Nikon. Neither Canon nor Nikon have published their TTL protocols (they want you to buy their stuff) but several folk have figured out how to emulate Canon and Nikon TTL protocols. If you use electrical sync then there are converters (Weikamp, S&S, Ike) that plug into your cable/hot shoe) and allow you to get TTL functionality from strobes that can speak that version of TTL. If you are using optically fired strobes then you need to emulate TTL (sTTL) for example. This typically works by sending a pre flash from the strobe to the subject then calculating the correct exposure then exposing the scene with the correct strobe duration.
Hope this helps
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