In the case of Oracle Entitlements Server (OES), this distinction is important. In Oracle's OES Product White Paper, a number of Gateways are listed as being able to use OES to manage authorization for Web Services. i.e. "interoperability". But only two are mentioned as being able to have OES directly embedded into the Gateway:
"To reduce latency, OES can be directly embedded into XML Gateways such as Oracle Enterprise Gateway and Vordel."
This direct embedding of OES into the Gateway is true integration. It reduces latency for customers, which is vital in environments such as financial transaction processing and telecoms. To see how this works in practice, check out the video below which shows how the enforcement of OES authorization policies is directly embedded into the Vordel Gateway: