Each time you start debugging a program, you are starting a debugging session. A list of your current debugging sessions is maintained in the Sessions view of the Debugger window. One session is the current session, unless no sessions are running. When you issue a Debug command, you are operating on the current session.
For more information on working with sessions, see:
Running a Program in the Debugger
Attaching to a Java VM
Debugging on a 64-Bit Java VM
Sessions View
Runtime Tab in the Explorer