So to be completely honest I have no idea, however I think it is a currently unused string.
It seems to be stored as a string, and when other clothes types are brought up (such as Sleepwear) it brings up a string of data.
In Rogues case for sleep wear it is a bunch of 0s, a 20, u'tank', and u'green panties'
In this case tank and greenpanties are variables that represent the stored data for those clothes types, and the numbers in the string I would assume coincide with what are basically "equipment slots" for the cloths.
While writing this I got the idea to test the string to see if I could store something in it. So you can store a string of data on it like it were cloths, but you could probably do that for anything and even if you do, there is no in game method of asking here to wear her "slut outfit."
So it is either not in the game yet if this is the case or its something you can't trigger. This would just be speculation however.
EDIT: Also as a bit of a warning, if you are setting variables, make sure to save in another slot because you can easily cause crash errors if you mess something up.