Release 5U1 Web Proxy Server is a maintenance release that fixes bugs in Release 5 Web Proxy Server. Release 5 Web Proxy Server, in turn , was a bug-fix release with respect to Release 4.
However, Release 5 Web Proxy Server includes better performance, more scalable architecture, better standards compliance, and a new administration interface as compared to Sun ONE Web Proxy Server 3.6 before its inclusion in the Java Enterprise System software.