The action window is the middle window appearing at the bottom of the Source Debugger.

The functionality of the action window is dependent upon the tab that is selected beneath it. These tabs and their purpose are:
BreakPoints: The BreakPoints tab displays a list of the currently set breakpoints, including the source file location, module, state, and statement in which they exist. The breakpoints tab also displays any currently "hit" breakpoints and the running thread that has been paused.
Use List: The Use List tab displays the source file location of each running steady-state statement that depends on the value of the variable currently selected in the module content window (i.e. the "consumers" of the value). Each statement so referenced will be triggered for execution when the contents of the variable instance changes.
Set List: The Set List tab displays the source file location of each running steady-state statement that is currently setting the value of the variable currently selected in the module content window (i.e. the "producer" of the value). In correctly written code, there should only be one such reference. Multiple such references result in a "double-set" condition and yields an invalid value for the variable instance.