Cache buster is %n which on each impression generate unique value. Also it burst the cache present on the system.The random number makes every ad call look unique to the browser and therefore prevents it from associating the tag with a cached file, forcing a new call to the ad server.
then append %n.So when user clicks, unique value will be generated which will be recoreded by third party ad server.Each click made by user in this session will be recorded but only single click will be count as valid.This methodology will insure that click fraud will not happen.
For some adserver cache buster in click tracker play very vital role.
In such kind of server if below is
In this case when %n is replaced with unique number and user click on the creative then unique number will be generated for %n present in click tracker also.
Impression will be counted only when these number (%n value from impression tracker and %n value from click tracker) is same then only impression count will be increamented.To Know more about cahce buster please refer