Generations in any form are sort of defined by the collectors. In Oakley's eyes, they don't often care, and even things like the A Wire and Straight Jackets were re-released under the same name, and as long as there wasn't a currently marketed model with the same name, there was no attempt to differentiate.
In terms of X-Metals, the most often 'generation' is the coatings on the Plasma and Titanium frames. For me, generations have to be defined by something concrete and tangible, which is why I never bought into the ruby generations. Aside from 'looks', there wasn't anything solid.
The frame coatings on original Plasma and Titanium were often ruined by sweat and moisture. They would turn black and discolored, so Oakley set out to fix them. Early releases had a more matte and gold finish, while modern is smooth and silver. Now this is somewhat concrete, but the things that makes it more official (and even to Oakley), are the serials. You will notice some ending in 'A' or 'B'. 'A' is gen 2, and 'B' is gen 3. Gen 1 has no suffix naturally since if they knew enough to add 'A' from the start, they would have just fixed the frames to begin with. At a certain point
barberp noticed that some colorways debuted with the new frames, and had no serial suffixes, so those never had generations. They were released late enough to not have the coating issues.
Now the second way we could define generations would be the recent Hydrophobic releases of Juliet and X-Metal XX. This again is concrete since the SKU's were changed, and the boxes had a new coin (in addition to price changes and the lens coating). I wouldn't consider the serials vs non-serials a generation, although some may argue that. Perhaps Serial is Gen 1, non-serial is Gen 2, and Hydrophobic is Gen 3. It would be a good argument, but really down to semantics at that point. Also not every frame had these changes, so it would be unique per model.
I guess in the end, 'generations' should really be specified as to what it references. Model or Frame. I'd never say Gen 1 Juliet, because per above, this could mean an early plasma frame, or anything with a serial. But some always had a gen 3 frame, even in the serial run, so it could cause a lot of confusion.
Confusion that's been an uphill fight for almost 10 years now, hehe.