Xdash answered most of it, but I'll add that you can switch your domicile whenever standing bid runs a bid (usually monthly), assuming there are vacancies at the base you want to switch to and assuming your seniority can hold it. But you won't know where the line holder cutoff will be until that month's line bids come out and are awarded. You can estimate where the bottom line holders will be for each domicile based on the previous months awards and use that to adjust your standing bid accordingly.

So you could eventually get PHX based (plan 3-6 months after IOE, sometimes sooner sometimes longer), find out when you would be a line holder in DFW, then switch your standing bid back to DFW when you are senior enough to hold a line there. Once you hold a line you can drop trips in DFW (pending enough reserve coverage) and pick up trips in PHX if that's what you are getting at. But you can't sit reserve in PHX and then grab a line in DFW as a PHX reserve guy. You have to be awarded a new base before you can bid in it. But as a line holder you can pick up open time from another base.