Since you are looking at the Camel JBI documentation I must assume that your routes involve jbi endpoints. You should probably know that Camel is capable of routing between many types of endpoints, jbi being one of them. Since you are using ESB 4, which is based on OSGi you have quite a lot of options besides jbi.
That said, the JBI spec section 4.1.2 describes the service model used by jbi, based on wsdl actually, where you have a service which is "a collection of endpoints that offer access to the same service. A service "implements" a particular service type (interface)". That means that camel will accept just the service uri if for instance the service only has one Endpoint, which is quite a common case, or if you don't really care which endpoint is used for the given service. However if your service has multiple endpoints you may want to use the full endpoint uri for disambiguation.
Thanks for the explanations.