Your GroupWise system exists in a world of email systems.
A Non-GroupWise Domain object represents a non-GroupWise email system. You can set up a non-GroupWise domain in your GroupWise system so that users and groups in the other email system can be represented in the GroupWise Address Book.
An External Domain objects represents a domain in another GroupWise system. You can set up an external domain in your GroupWise system so that users, resources, and groups in the other GroupWise system can be represented in your GroupWise system. In addition, the other GroupWise system can set up your GroupWise system as an external domain as well. When both GroupWise systems have external domains to represent each other, the External System Synchronization tool can keep both GroupWise Address Books in sync as users, resources, and groups change over time.