Supposedly, the higher the number, the more chance that unit will appear (and in greater numbers) in that event. If you notice from the ReligousFanatics line, Crusade is at 20, which means you will get a whole lot more of them in a Crusade than Order Foot Soldiers (-15).

As for the events themselves, they are pretty self explanitory. Crusades and Jihads are just that, Cath_Loyalists is what happens when you invade a Catholic province then immidiately move out and let it go rebel back to its former owner. The various Rebels is what happens when your loyalty rating in the province drops to a certain amount and the province rebels and a stack of rebel troops appear. Honestly, I dont know what Bandits and Peasants mean, as I havent tested it.

Obviously, a unit can only appear in a rebellion in a province they can normally be built in (IE: Highlanders in Scotland)