IMAP maintains message state on the server through flags.
Common system flags include \Seen, \Answered, \Flagged, \Deleted and \Draft.
Because multiple clients can access the same mailbox, flags are part of synchronization rather than purely local UI state.
Provider-specific labels, categories and archive behavior can sit above or beside standard IMAP semantics, so migrations should test the user-visible result rather than assuming every product maps folders and flags identically.