Raid stacking refers to building a raid that has optimised (ideally all) buffs from a wide variety of classes and specialisations. The aim with stacking is to provide all the buffs listed by having at least one of every class listed under each buff. In addition to this, most high end raiders theorycraft around having 100% raid stacking (i.e. in 25 man raids), allowing them to maximise their stats by moving points from otherwise spent on buffable areas- thus, it is important to know what classes to invite as to have rounded composition and buffs.