Using the Designer Screen –Advanced - Learning Informatica PowerCenter 9.x - page 58

Read the points clearly before you proceed and click on Next .
3. The next screen will ask you to select the session task for the mapping you wish
to debug. Select the session task you created in order to execute the mapping.
Even though running a debugger is a manual process, you still need a session task
because Integration Service needs to get the path to extract and load the data that
is provided in the session task.
Note that you can use an existing session or create a new debug session to run the
debugger. If you use an existing session, the debugger will use the properties
mentioned in the session. You need to mention all the details if you create a new
debug session.