Since this forum seems appropriate, I'd like to know. Show of hands and all. XP award advancement bases itself on the GM awarding the player XP or some other moniker for specific actions such as killing or discovery. Chance per use advancement has a chance to increase a skill/attribute when used. And there's also a mix of the two. An interesting example is Risus, which mixes the two by awarding die rolls for a chance at advancement for any cliches moderately used during play. Another example of mixing is when skills are given XP individually each time they are used, but that constant record-keeping personally annoys me. One "on use" advancement that I haven't seen is re-rolling a natural 20 against the skill/attribute used, increasing it upon surpassing it.