Without more information, my "best guess" for the recommendation would be with regard to the Microsoft Exchange Server 2007 "Always On" solution. This solution is a clustering solution (CCR) where there is a public network connection (virtual) of the active and passive servers and connection for the heartbeat between the active and passive servers through a private network connection with separate network-addressing scheme.
A couple of links I found that goes into more discussion regarding CCR:
Other than that, I would need more information with regard to the "why" of the recommendation to be of more assistance. Does this help you?