From what I remember, in it's current form, there really is none. Either the ship with the emmissive armor takes no damage (from the small number of weapons that do less damage than the emmissive ability) or it takes full damage. It does not reduce the damage from more powerful weapons. Even fighters will not be stopped by it, because all the fighter weapons in a group stack. You'd be better off using crystalline armor.
Someone correct me if I'm wrong cause if I am missing something I'd like to know.
My guess is that emissive armor was intended as a protection against low tech races and small weapon types carried on fighters. Good idea! The only problem is that most ship weapons can overcome the emissive armor's protection and do FULL damage. A group of fighters fires as one big weapon (all the damage is added together then applied) making the armor once again useless. If each fighter fired individually then only fighters with 20+ (or whatever the bonus to emissive armor is) would do damage.
Emissive Armor is supposed to reduce the damage done by every hit up to the amount set in the ability for the given component. Unfortunately, there has been a bug in it since day 1 where the damage reduction is skipped if it exceeds the ability maximum. This makes Emissive Armor an expensive toy that can make you immune to low-tech weapons but just makes you pay more for your normal armor against higher tech weapons. Hopefully this will be fixed some day. There are so many little things like this to get to that it's hard to guess when...