I always kinda wondered about that.

Now, since you seem to know a lot about this, I'm pretty sure I've seen it happen that T+ was sold out on a certain route while T was still available.

When seats are allocated into fare buckets, is it a fixed allocation that's done ahead of time, and then maybe adjusted manually later, or is the system always adjusting the allocation? It seems to me that if there's a T fare available, and I want to buy a T+, it would make more sense to convert the T seat to a T+ seat and let me buy it. In other words, when the number of seats allocated to T+ is sold, allow customers to buy T+ seats from the T inventory, rather than showing T+ as sold out. Or is the theory that customers looking for T+ will go up to L rather than back to T?

It just seemed strange to me that the higher fare classes could sell out before the lower ones.

Thanks, BTW, for all the good info so far. Much appreciated.