OOUI does not document the 'back' and 'close' flags anywhere in its documentation, for dialogs, action sets or flags. For example, all other flags are documented here, but not those two. Action sets are documented here but don't mention these (newer) flags.

'back' was introduced as part of T153863: Upstream VE mobile dialog functionality, 'close' as part of T225021: Support a new "close" flag (distinct from "back") in OOUI dialogs