Complete Arsenal is the ability to possess all kind of powers and abilities within a known verse. This means that the user is capable of using all known powers in their verse including even ones that have yet to be made.
Users with this power, when fully mastered and having extensive knowledge on all known and yet to be known powers (i.e the nature of each powers, it's strength's and weaknesses, what effects it can do when using another power with one, etc.), makes them formidable as they have many ways to counter any potential threats or fights with enemies, what possible combo's they can make with any powers they can think of, and so on.
There is also Nigh-Complete Arsenal, which is the ability to have almost all known powers in the verse. The main difference between this and the Complete one is that while a user with Complete Arsenal is capable of using all known powers in their verse including ones that have yet to exist and such, the user with Nigh Complete Arsenal may only be limited to using all the known powers that exist in the verse, but can't use anymore than that such as powers that have yet to be created.