SCR in exchange 2007 is used to replicate the exchange database to the remote site. Usually, this kind of setup is done for site disaster recovery where the exchange database is replicated from the source mailbox server to the target mailbox server.
The scenarios that are supported for SCR are:
The source can be any of the following:
Standalone mailbox server.
Clustered mailbox server in a single copy cluster (SCC).
Clustered mailbox server in a CCR environment.
The target can be any of the following:
Standalone mailbox server that does not have Local Continious Replication (LCR) enabled for any storage groups.
A standby cluster, which is a failover cluster where the passive clustered mailbox role is installed, however no clustered mailbox server (for example, no Active Clustered Mailbox role) has been installed in the cluster.
To install Mail Security on source and target servers