We call the task of replicating between two different database groups external replication.
There is actually no difference in the implementation of an external replication and a regular replication process.
The reason we define them differently is because of the default behavior of a cluster to setup well connected database groups.
This may be limiting if you wish to design your own replication topology and external replication is the solution for those unique cases.
In RavenDB we introduced a new kind of replication, delayed replication, what it does is replicating data that is delayed by X amount of time.
The delayed replication works just like normal replication but instead of sending data right away it waits X amount of time.
Having a delayed instance of a database allows you to "go back in time" and undo contamination to your data due to a faulty patch script or other human errors.
While you can and should always use backup for those cases, having a live database makes it super fast to failover to and prevent business lose while you take down the faulty databases and restore them.