To receive information about events related to users connected to the Sendbird server, add a user event handler with its UNIQUE_HANDLER_ID by calling sb.addUserEventHandler().
If you want to stay informed of changes related to users and notify the changes to different client apps, define and register multiple channel event handlers using the UNIQUE_HANDLER_ID.
There has been an update on the total number of a user's unread messages because the user read messages in group channels.
The user's devices running client apps. The devices are notified with the total number of unread messages and the number of unread messages by custom channel type.