This document explains using the %ZZCUSTOM% and %p macros to pass a custom string through an ad tag dynamically at runtime. %p is the single line equivalent of %ZZCUSTOM%.
The custom string is passed to the ad through the variable p as shown below:
For example, if the user wants to pass the clickthrough dynamically at runtime instead of hardcoding in the ad, then he passes the clickthrough (http://www.google.com) through the placeholder (p) in the ad tag. Whatever gets passed through p will get replaced at each and every occurrence of %ZZCUSTOM% in case of HTML uploads OR %p in case of IFrame uploads.