The Voice Connector has its own release notes that give a brief explanation of what it is used for. Here are the release notes for the latest version:
http://www.cisco.com/en/US/docs/voice_ip_comm/unity/voice_connector/release/notes/mig/vc1103rn.html
The Networking Guide for Cisco Unity also has an overview of the usage of the Voice Connector. Here is the 5.x version:
http://www.cisco.com/en/US/docs/voice_ip_comm/unity/5x/networking/guide/ex/5xcunet010e.html#wp1065351
If you look at the AMIS or VPIM chapters in the same guide, you'll also see more info there about the role that the Voice Connector plays for those types of networking. If you're interested in Bridge Networking, the Bridge chapter has a link to the appropriate guide that contains info about the Voice Connector usage for the Bridge.