Upstream users are users publishing audio/video. A user becomes an upstream user after publishing a stream by calling publish() and is no longer an upstream user after unpublishing the stream by calling unpublish(). However, if a user calls muteAudio() and muteVideo() to disable local audio and video, the user is still considered an upstream user, even though remote users can no longer receive his or her audio or video. This is because the user continues to publish data packets, although without audio and video, to the backend server.
Limit on Upstream Users
The TRTC backend limits the number of upstream users in a room to 50. If there are already 50 upstream users in a room, the 51st user trying to publish a stream will fail. To allow the user to publish his or her stream, your application can ask one of the 50 upstream users to call unpublish() to stop publishing.