Whether a secondary replica is currently connected to the primary replica:
0 : Disconnected. The response of an availability replica to the DISCONNECTED state depends on its role:
On the primary replica, if a secondary replica is disconnected, its secondary databases are marked as NOT SYNCHRONIZED on the primary replica, which waits for the secondary to reconnect;
On a secondary replica, upon detecting that it is disconnected, the secondary replica attempts to reconnect to the primary replica.
1 : Connected. Each primary replica tracks the connection state for every secondary replica in the same availability group. Secondary replicas track the connection state of only the primary replica.