vCenter automatically chooses two heartbeat datastores per host (can have an overlap between hosts). When the host is network isolated or partitioned, the datastore acts as a witness to rule if the host has truly failed or is network impaired. This setting may seldom require to be changed, if there is datastore known to be more resilient then the user can override vCenter's choice and configure the heartbeat datastore manually. An even safer option would be to prescribe a heartbeat datastore and complement it with an automatic choice, which is the third option available.