How to interpret AlwaysOn event hadr_db_partner_set_sync_state We have a program that listens to the SQL Server AlwaysOn_health event session
for state changes.  When we simulate a secondary server going offline or online, we get the hadr_db_partner_set_sync_state event from SQL Server.  (See below.) When I Google for "hadr_db_partner_set_sync_state sync_state" I get 6 hits and none explain the fields of this event.

{"EventName":"hadr_db_partner_set_sync_state",
 "Fields": 
 {"database_id":"95",
 "commit_policy":"WaitForHarden",
 "commit_policy_target":"WaitForHarden",
 "sync_state":"NOT",
 "sync_log_block":"28119150887390",
 "group_id":"070e4751-d4cc-4154-9ff5-d2723838d0e4",
 "replica_id":"8b1c6688-cd6d-4668-b90b-f235121c82b2",
 "ag_database_id":"16e2a407-e136-4755-85b4-9f576d542ed1"
 },
 "Actions":{}
}


Which combination of fields in this event tells us a secondary server has gone offline? And which tells us a server has been restored to online?

If there is some other event we should be watching for, that would be helpful also.
According to How It Works: Always On–When Is My Secondary Failover Ready?, a commit_policy of

DoNothing 

is an indication that the secondary is offline.  A commit_policy of 

Delay 

indicates that the secondary is online, but catching up.  And finally, 

WaitForHarden 

indicates the secondary is caught up with the current transaction.