CDU, is short for Countertop Display Units and in these you can find 1.5 sets of the  collection and at least 1 complete set of a particular collection. So if you're someone no patience, no appreciation for surprises, and just want to get things done and over with, make sure you get them all by snatching the whole CDU box. Also, more importantly and as if it wasn't already a big deal, by getting the whole thing you're also guaranteed to receive at least 1 secret special character.