Stages

Hi, I would like to know how stages work, I know it will depend in who's selling the stages, but I would like to know what is modified ( pharameters) in stage 1. (Just some efficiency volumetric changes, and limiters? and stage 2 because stage 3 reflash everything must be modified or changed right? And how do u set a target a.f.r for stage 1,2,3 , (iddle a.f.e and full load a.f.r. At the moment just saw some webinars and free tuning lesson but trying to get the vip package this year, thanks in advance.