Diagnostics is a separately licensed HP product to is composed of a Commander, a Mediator, and one or more probes and collectors. When properly setup, it gives the user a total view of what is happening on monitored system. For example, it will give the Java calls by number of calls sorted by call and the same thing for Oracle, and etc. Once you are licensed to use Diagnostics and have properly set it up, you can connect the LoadRunner Controller to Diagnostics and view what is happening at the atomic level on the servers that are under test. I recommend that you go to HP's Web Site and read the product white papers for a further understanding.