Traditionally, fare-class inventory worked like Russian nesting dolls: If there was inventory in a lower fare class, there would be at least as much inventory in every higher code. Y9, B9, M9, E8, etc.

But over the past year or two, I've noticed that United is imposing strict capacity controls on B-class fares. The most extreme example I've seen was yesterday, when a domestic flight was something like Y9 B1 M9 E9 ... L4 K1 G0 XN4 X0.

My best guesses are that this is related to Premium Plus or that United has corporate contracts that substantially discount B-class availability (as the cheapest "full fare" code) and United is trying to limit that discounted capacity.

But does anyone know for sure?