Nobody who knows how it works would post here; revenue management is a closely guarded trade secret at each airline.

The basics are simple: they use past load information to predict future sales; they lower prices when sales trail expectations, and they raise prices when sales exceed expectations. Beyond that, it's a giant black box.

Important routes may be managed directly by a human; event dates have to be put in each year (example: when the Super Bowl is announced, somebody notes that into the calendar for the host city; when the dates for Austin City Limits are announced, their Austin person adjusts accordingly). Most are managed by computer.

When I write "United thinks they can sell all of the seats" on an upgrade thread, that's an inference from the inventory numbers. First/business inventory is J (JN) C D Z ZN P PN PZ IN I. The maximum number of seats in the inventory data is 9, so there's no way to tell the difference between "9 seats available for sale" and "40 seats available for sale." However, the further right in the inventory list the 9s go, the more discount seats UA is willing to sell. So, J9 JN9 C9 D9 Z9 ZN9 P9 PN0 PZ0 IN0 I0 means "no upgrade (PN/PZ) or award (IN/I) space available, but because it's P9, a waitlisted upgrade might be possible." On the other hand, J9 JN9 C0 D0... says "UA thinks that they can sell the rest of their inventory as J fares, the most expensive seats they have." And, of course while J9 could be 9-60 seats, J8 means exactly that: 8 seats left to sell. So when inventory comes back as J6 JN6 C3 D1 Z0..., it's not looking good for an upgrader, as there are only 6 seats available and no discount fares. (The inventory isn't additive -- in that example, there are 6 seats available, but UA is willing to sell one as a D fare, in which case the inventory would probably to to J5 JN5 C2 D0... or three as a C fare, in which case it'd go to J3 JN3 C0...).