Board Thread:Questions and Answers/@comment-6011882-20130308175404/@comment-5933544-20130309171900

Well when a new event starts, the new Raid UR will have bonus effects on all of that event's raid bosses. Then Hidden bosses get released, and a new Raid UR gets also released with bonus effects specifically to those bosses. Finally, secret bosses get released and another UR becomes available along with it. This is how you figure out which URs belong to what.

Then after that event ends, all you really care about is the Med-damage increase, so it doesn't matter even if you don't know what bosses, say Rhea has a greater effect against.