SSB user who accesses the room server and is considered internal because they have already joined the room and may even have registered an alias in the room.
Definition: an internal user of a room is any SSB ID for which the room grants a tunnel address. In other words, if an SSB ID is reachable via a tunneled connection through a room server, then they are considered an internal user of that room.
Becoming an internal user: read more about that in Joining a room.