weblogic.xml会话复制区别?

# memory—Disables persistent session storage.
# replicated—Same as memory, but session data is replicated across the clustered servers.
# replicated_if_clustered—If the Web application is deployed on a clustered server, the in-effect persistent-store-type will be replicated. Otherwise, memory is the default.
# sync-replication-across-cluster—The replication will occur synchronously across the cluster.
# async-replication-across-cluster—The replication will occur asynchronously across the cluster.

请详细解释什么时候使用replicated和sync-replication-across-cluster以及这两个的区别？