the concept of Zones and Joins in UM is similar to Broker territories and gateways.
Zones are like Broker Territories, where messages are automatically forwarded to another member of the Zone if there is at least one interested subscriber there. This forwarding does not take into account any subscription filter the subscriber may have (same as Broker). The only difference with Broker is that with UM you will need to create JMS topics (or sync IS doc types) against every UM server in the Zone, whereas with Broker you only needed to do that on one member of the Territory.
Using Joins, you can control the flow of documents between two UM servers. Unlike with Broker, a UM server does not need to be a member of a Zone before you can establish a Join.
To establish a Join, you must first “add” one realm to the other. You do this once, and it establishes the basic bidirectional communication. Then you control which doc types (channels or topics) you want forwarding across the Join by creating a Join on each one. If you want documents to flow in both directions, then you must configure two Joins - one in each direction. You can also set filters on Joins, just like you can with Broker.
Hope this helps.
Software AG Product Management