This Counter32 counts the number of times an outgoing SNMP message was dropped because the SSH session it was supposed to use was no longer available. Admins use it to detect messages lost due to session teardown or timeout happening between message queuing and actual transmission, a different failure mode from an outright failure to open a session in the first place. For example, a rise in this counter alongside stable session-open counts might indicate sessions are timing out or being closed prematurely while messages are still queued to use them.