Add detailed information about fault-tolerance.

Author: tselmegbaasan

modules/ROOT/pages/clustering/monitoring/status-check.adoc

@@ -44,3 +44,14 @@ can either mean that there is a real issue in the cluster (e.g. no leader) or it
 
 In general the `replicationSuccessful` field can be used to determine overall write-availability, whereas the `memberStatus` field can be checked in order to see whether the database is fault-tolerant or not.
 
+[NOTE]
+====
+Members that are `REPLICATING` are good from a data safety point of view. They can participate in replication and keep the data durably until application. They are also up-to-date and therefore eligible leaders. So they add to the fault-tolerance.
+
+Members that are `APPLYING` have all the qualities of `REPLICATING` members, so they too add to the fault-tolerance. But they are also applying to the database, which is a requirement for writing transactions and reading with bookmarks in a timely manner.
+
+Lastly, `UNAVAILABLE` members are either too far behind or unreachable. They are unhealthy and cannot add to the fault-tolerance.
+
+====
+
+