The high watermark for server throttling. If throttling is not engaged and the number of messages that are held by the server ...

The high watermark for server throttling.  If throttling is not engaged and the number of messages that are held by the server for processing rises above this number, throttling will engage on a per-connection basis.  While throttling, no new messages will be accepted on connections that have outstanding messages in the queue until at least one of them is processed or the total number of messages drops below the low watermark.