Why Is United Restricting B-Class Inventory?
 

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?

do you have some examples?

B has recently been used to sell E+ fares at a modest premium above the lowest coach fare in many markets, similar to how DL markets and sells C+ as a pseudo-separate cabin. It could be that UA is managing inventory and revenue as such, rather than as if B was a full fare bucket just below Y.

This is precisely what they’re doing on some routes. I suspect if OP were to look at the seat map, there would be precisely one E+ seat available.