The note as plain text: 1 to 10,000 characters, and not only whitespace. HTML isn’t interpreted.
A key you choose, up to 100 characters. A retry with the same key on the same thread returns the first note and adds nothing. Once that note is deleted, the key throws BAD_USER_INPUT.
TeamMember ids to notify, at most 20 different ones; a repeated id counts once. Each must be on the store’s team. They get the same in-app, email and push notice as an @mention in the help desk, following their notification settings.