Experience points can be awarded for three different things.
PRIME REQUISITE BONUS: The total XP awarded is modified by the PRIME REQUISITE level of the character, i.e., the Ability Score tied to his class. Fighters need STR, Magic-Users need INT, Clerics need WIS, and Thieves need DEX. Combination Classes use the lower of the two Prime Requisites to find their bonus. If a character has a Prime Requisite of 3-6 he suffers a -20% penalty to his experience award. If a character has a PR of 7-8, he suffers a -10% penalty to his experience award. If a character has a PR of 13-14 he gains a +5% bonus to his experience award. If a character has a PR of 15-18 he gains a +10% experience award.
Some monstrous species, e.g. Bugbears, receive a penalty to experience awards (e.g. -20%), apply the penalty after all other calculations are made.