You have a superheroic identity and a costume that clearly identifies you. You almost always wear this costume (or some variant thereof) when in action. This is sort of the heraldry in the present day; a super costume can look any which way, but it is clearly recognizible as such, generally has a distinct color scheme and a recognizible badge. It serves to identify the wearer from a distance.
A costume need not actually be a costume at all; for some supers their natural form is conspicious enough. However, if you wear any clothes at all, they should be specific.
This limitation is clearly less severe than most others. It exists because costumes is something I want to encourage.