Some people desire to have replica set internal traffic (heartbeats, data transmission) use different networks or routes than client-to-member communication.

One way to achieve this, which has been observed to work in the field, involves provisioning replica set members on hosts having multiple network interfaces, and configuring the host name resolution among the hosts where the replica set members run to use one set of addresses, but having hosts where clients run use a different set of network addresses. (And then being sure not to run any client applications on the hosts where the replica set members run, of course.)

If this mode of deployment is intended to be a fixed property of what people can do with replica sets, it should be tested for, in case any subsequent changes might break it.