A HADR_SYNC_COMMIT wait occurs when the session is waiting for a transaction to commit or rollback when the database is part of a Database Availability Group (High Availability.)
A transaction is considered committed when hardened (written) to the local database log file and when hardened (written) to the configured number of secondaries to establish the quorum level. The transaction needs only to be hardened (written to secondary database log file), not recovered on the secondary.
The Log Sequence Number (LSN) is used to track hardened database log records. As soon as the hardened LSN advances to include the