Swap Costumes are alternate costumes for the characters of Street Fighter X Tekken, available as part of a DLC package.
Each side's costumes are inspired by the other, i.e. alternate costumes for Street Fighter characters are based off Tekken, and vice versa. Several of the Swap Costumes directly copy - or else are inspired by - another character's costume. Capcom's official Japanese website has a rumor section accompanying several pictures, providing the source of inspiration for each costume.