And where can I find documentation about these format codes?
#What are "%p" and "%2.s"?
iron briar
zealous bane
%p is progress in the form of a timer. Think of the remaining duration of a cooldown, or the remaining time on a buff. It follows the order of your triggers. Thus if you have a buff trigger first, it'll show that duration, if the buff is gone, and your second trigger is a cooldown trigger, it'll show the remaining duration of the cooldown.
%2.s is the thing you do if you want to see the stacks of only the 2nd trigger.
If you click the little icon on the right side of that box, the square with the list on the right, you see all the options you can use, with the triggers that are present.