Not far off. Basically, they cancel your preferences beforehand and assign whatever they need.

It was intended for something like: everybody wants and bids for Christmas off. There’s a lot of flying to be done around Christmas, and obviously not everyone can have it off. More junior pilots get “unstacked” around Christmas (named for the “stack” of trips that need to be flown then, which are literally displayed on top of each other in certain programs), and the program gives those junior pilots trips during when they had asked off.

That’s fine and well, but the problem arises when an airline isn’t staffed properly. Suddenly tons of people, even the most senior pilots, are getting unstacked randomly and frequently. It’s a big hindrance to quality of life.