The multi - server / cluster add on gives BOC users on one cluster or server platform, visibility of users on another cluster of platform. This Add on allows the connectors to share call information with each other, and then with their local BOC clients. Once this add on is enabled that BOC connector can begin to gather call information from other BOC connectors. A requirement will be an overriding directory containing users on both servers or clusters, like Microsoft Active Directory. For 2 (or more) way communication you would need to have this Add on each server endpoint that you desire the local users to have cross server or cluster visibility.
Why would an organization have multiple clusters
Company size is too large to be handled by a single cluster.
Company acquisitions leave you with a mix of different platforms for employee UC.
Geographical constraints require you to split your employees into separate clusters.
Bridge Operator Console is a replacement for Cisco Attendant Console. It offers a more robust feature set, including Lync integration, at a much lower cost. You can also replace Fidelus, replace ARC Soltuions, and replace IQNet.
Bridge Operator Console is a replacement for Microsoft Lync Attendant 2010. It is superior to Attendant Pro from landis, Competella, and Enghouse