At UA on reserve you get 12 days off in a 30 day month and 13 days off in a 31 day month.

If your assigned airplane/base combination doesn't have any Intl flying (defined as Europe/ Asia/ beyond 15 south in South America) then you will get 1 day off that is "movable" by the company (they call it a Flexible day off - isn' that cute). All the rest of your days off are not movable by the company. So for the domstic reserves that means that you have one day that can be moved by the company if they have no other options. It can only be moved if there are no other reserves that are available without moving the day. It also can't be moved for a trip that starts on the original day off. So they can't give you a 1 day trip on that day off and just move the day off somewhere else. If tomorrow was your FDO they could give you a 2 day that leaves today and gets back tomorrow.
If your assigned airplane/base does have intl flying then you will get 6 HDOs (Holy Days Off) that can't be moved by the company and the rest will be RDOs (Regular Days Off) and 1 FDO. The RDOs are different than the FDO in that the only way they can be moved is if the trip they are trying to cover is an intl trip and all the other restrictions of an FDO move are met.

In either scenario the day off that was moved is always "restored" on your next scheduled work day so you don't end up with less total days off. Your number of days off is "guaranteed" but (in rare circumstances) 1 (or more for Intl) of the eligible days off could be moved.

Right now all the 737 and A320 bases are domestic. SFO, LAX and DEN are domestic on the 767. ORD, IAH, DCA, and EWR are Intl on the 767. All 777 and 787 bases are intl.

Again, this Intl Reserve setup is a bad situation, but to a certain extent it comes with the territory of having the largest WB fleet. Intl cancellations are extremely expensive so the company wants/needs this flexibility. Even though my experience has been that it is not used very frequently, it does suck in the QOL world. With that being said I think the company is going to have to pay a very steep price to keep this level of flexibility going forward as this is one of the most hated parts of the Intl reserve system. I suspect/hope that either the number of days that the company gets every month that can be moved will go down or a financial reward will be paid to the pilot that gets their day moved will be incorporated in the next contract, but I doubt that this will ever go completely away.

I think most WB reserves that live local will tell you that being local on WB reserve is the best gig going and that you will spend more time at home than you ever have in your aviation career. On call yes, but at home. I think that most WB reserves that are commuters will tell you that being a commuter on WB reserve is the absolutely worst gig going and you will spend more time in your crash pad than you ever have in your aviation career!!!