We have movable and immovable days off. If you are available for the entire month, you get 12 days off with 8 of those being immovable days off and 4 being movable days off. The 8 days of immovable MUST be in at least two blocks. You cannot bid for them all in one block because PBS will deny the request. So you need to bid a minimum of a block of 2/6 (or 3/5, 4/4). Can't have 1 immovable only.

There is a proration table listing the days off you get and how many immovable based on your days of actual availability. As an example, have training? Training counts as unavailable days. So if you have TRVL, SIM 1, SIM 2, then you are unavailable 3 days of a month and you are prorated down to only 10 days off in a month with I believe 7 of those being immovable. Vacation works the same way.