“In Principle these two garments refer to the same reality, and yet they do not have the same structure,”
It is fair to say that fashion can be an idea. Designers, or creators, draw pictures, forms, or ideas, in their imagination, or in their own world. This “reality” Barthes mentioned is indeed not a “reality” but an origin of the fashion, the idea. Fashion is an idea and an actual garment, illustration, verbal description and even photography or video is all form which expresses the idea, it bases off.