Most menu entries may have additional self-explaining parameters, which may be checked. If some of these parameters are not available for some kind of action, they are disabled.
Here is one example for executing an external program:
You can see that a
perl script is called, which should work with current selection. The Needs selected text parameter is checked to assure a selection. The output of this script will be inserted (Insert the output of the chosen program) and replace the current selection (Replace selected text), but not selected itself.