1) If you fly a GS on your RES days you get PB days.
2) If there are enough RES days left on your schedule in the month, the PB days start on the RES days immediately after your GS.
2) If there are not enough RES days left in the month they go into a bank (which you can check at the bottom of your timecard.)
4) You can go to iCrew-->PCS-->LEAVE REQUESTS-->PAYBACK DAYS, and use them through the PCS process. Unfortunately they have no more priority than a regular PD so you often run into a CAPPED RES day issue. Otherwise, you can drop a trip with pay equal to the number of PB days. The obvious strategy would be to drop a high-time trip using PB days. If you are on RES the PB days would pay less (since RES days pay less -- 3.75 for instance.) As iFlyer wisely pointed out it's best to save them for use on a REG line.
5) If you can't use them by DEC 31 they are put into your VACA bank which means they have even less value (although I've heard you can still try to use them prior to the new VACA year.)

A wealth of knowledge to be found here --> https://dal.alpa.org/DesktopModules/...d=0&TabId=2593

I really wish they would tell guys about the SRH in INDOC.