How does Apple Mail actually track coloured messages flags?

**NB. I'm purely asking out of curiosity, flags sync correctly for me on both iOS and macOS using a GMail account.** In macOS/iOS Mail you can assign different coloured flags (orange, red, violet, blue, yellow, green, grey) to messages, rather than just starring them. How is this stored, is there something in the IMAP specification that supports extra metadata for messages? To my knowledge, Gmail has never offered anything other than starring messages, although I seem to recall there may have been an optional extension that supported extra colours. (I assume iCloud syncing isn't involved.)