Join messages announce you to the gamemode when you log in or arrive from another gamemode. Eight are live, and you pick one at a time from /joinmessage.
Join messages are custom announcements that appear in chat when you join a gamemode. Only players who equip one are announced; nobody gets a join message by default.
The message is shown to the players of the gamemode you just joined, about 1 second after you arrive.
It appears when you log in or enter a gamemode from somewhere else, for example from the hub to Skyblock, not when you move between servers of the same gamemode.