A SIP (Session Initiation Protocol) gateway can be used to connect a Sametime deployment to another service, whether that be another external Sametime deployment, or a completely different service (AIM, Yahoo, Jabber).
This topic is incomplete, so I've just left my notes in place below. Please understand that this is all drawn from observation and speculation, not from actual knowledge of the inner workings of the SIP gateway(s)
- Why doesn't the SIP gateway use the community field in the user ID block instead of an "@E " prefix and a federation-mapped suffix?
User ID Routing
The SIP gateway will be used for awareness and messaging with users marked as external. External users are indicated by the simple expedient of prepending the three character string "@E " to their identifier. In effect, when talking to an external user, you're talking to the SIP gateway, which is then translating the conversation to SIP and routing it over a federated connection to another server. User routing is done based on suffix matching of the user identifier. For example, identifiers ending in "compand1.com" will all be routed via the same federated connection. If a federated connection cannot guarantee uniform suffixes on identifiers it handles, the gateway can be configured to effectively create a virtual suffix. For example if company2 used simply a user's name as the identifier, the suffix ".company2.ext" could be appended to help mark those users for routing. Such a virtual suffix would need only be unique to that particular SIP gateway.