LordGigaIce, that's basically what I said, I thought? Except for the bit about the retailers; I think my idea there was I remember it being done similarly to how Phazer mentioned, but have no way of knowing how close to that or far from that it is now. I think all we're left to quibble over is whether to call Generations an umbrella or line and Studio Series/AOTP lines or sublines.
To be honest, I spent a silly amount of time trying to find WHERE it said "Generations" on the packaging, thinking it was text or something near the line logo somewhere. And when I finally saw it was the Autobot logo with "Generations" under it -- and my eye had been glossing over it, because it's really not that different from the Hasbro or Takara logos, or the "authentic Transformers product" notation, the current "# steps" graphics, and so on. Reduced to that kind of indicator/notation -- and not appearing with the line logo -- I think it's fair to think of it more as a category of toy rather than a line itself.
This is consistent with how they don't typically refer to these or necessarily promote these AS Generations alone (typically paired with one of the actual line names) and we typically only see it in that little logo and in internal documents like solicitations and computer listings.
But again, that probably just comes back to splitting semantic hairs, so I'll just shrug