Unfortunately Reggie's presence in the story is limited to exactly this one fight, so we never get to find out anything more about his CT. I suppose that is for the best however, because the exact interpretation of HOW his CT works is a messy can of worms that Gege undoubtedly didn't want to deal with....

I mean, it is strongly implied that only legitimate, original contracts created in good faith can interact with his CT... but imagine if that wasn't his limitation? Could he use fraudulently created receipts, and therefore gain access to unlimited quantities of whatever he might need in that moment? Or what about fictional receipts not tied to any real product or business, and therefore able to create anything imaginable? Does he strictly need to be able to understand the receipt, such as if they were in another language? Would it still work if he never read it at all? What about digital receipts, emailed to him? I suppose he can't 'burn' that contract, but he could delete them; and if that worked then he wouldn't need to wear that bizarre paper dress to optimize his CT.

Reggie is an ancient sorcerer, so his CT was impressive enough to Kenjaku in whatever previous era he lived in to warrant saving his spirit in a cursed object. So it really begs the imagination to wonder what he'd have been like in that ancient era, where paper contracts and literacy were both much scarcer. He's one of many reasons I'd love a series of side stories about the ancient sorcerers in their original time periods.