Looking for reference RE: events & variable passing

Does anyone know of a good reference for all the variables passed when an event like, say, SPELL_AURA_APPLIED fires?

I’m trying to repair an addon that broke hard after the 11.0.2 update (but not the prepatch, which…weird) and unfortunately the original author only referred to some important pieces at the very end by reference, not by name, like:

auraType = select(4, …)