A: Heartbeat datastores in vSphere 5.0 are intended as a secondary path of communication for vSphere HA should a host's management network connection fail. Heartbeat datastores, in conjunction with this extra communication path, protect a vSphere HA cluster against that failure creating a host isolation situation.
Heartbeat datastores work best when all hosts in a cluster share access to at least one datastore, although it isn't required that they do. Some won't have shared access, based on implementation-specific requirements. If your datastore configuration is such that not all hosts share at least one datastore, it's recommended that you create another small datastore that is shared by all hosts for the purpose of hosting the heartbeat datastore.