This read-only Integer32 is the session-level receive window size the peer advertised when the session was created, capping how many payload packets can be sent to that peer without an acknowledgement, with a non-zero value implying the peer has requested flow control. Comparing this to actual send rates helps an admin see whether a session is being artificially throttled by a small peer-advertised window. A value of 8 here on a session experiencing choppy voice quality would point to that small receive window as a likely contributor.