Okay dude, I don't really get your question but I'll let you know the terminology.
getServetPort() basically will return the default port that the server is listening on. So for example, suppose you got Tomcat 5.x/6.x runnin. The default port (out of the box) is 8080. This is the port, teh server is actually listening on.
getLocalPort() is the actual port being used to service the request. Think about one million concurrent requests being sent at the same time. The container (tomcat/websphere/jboss) will internally use a port to actually service the request...why? To handle concurrent requests at the same time.
Remember one servlet, many threads. Ports are communication channels between the container. the app and the OS. They are reserved internally by the container to distinguish each apart. So if concurrent requests are being served, the container will internally use a specific port to service thread A and B.