It's a staffing thing. If the reserve buffer is at, say, 18, that means on any given day they want a surplus of 18 reserve pilots working. By surplus I mean... If they have 2 trips that have been unassigned (so they're in "open time"), they want to be able to have enough reserve pilots to cover those 2 trips, and then still have 18 extra, meaning they'd want 20 reserve pilots working that day.

The implication is: the higher the buffer, the more reserve pilots they need. If they have less than the buffer, reserve pilots can't swap that work day for another and line holding pilots can't drop a trip that includes that day.

Basically, it's awesome when the buffer is low and the staffing is high because then all the pilots can do whatever they want. It's sucky when it's the other way around.