Costume Slots most likely. I have half a dozen, but I've found that this number isn't as big as it may seem at first after you have filled the "essentials" (for my Vulcan: duty uniform, dress uniform, traditional robes, civilian clothing, mirror universe uniform).
It barely leaves anything for "occasional" clothing such as a TOS dress for time travels or a slightly more militarized away team suit for Borg STFs.
That said, if they do it like in CO and dump tons of new stuff into the C-Store at launch, I may have to reconsider. Could well be that there's some nice new clothes available. Perhaps even something for the Klingons!