The Volume tool is a slider that you can use to control the volume of MIDI output. Moving the slider will generate volume controller messages on all 16 channels.

The button immedately below the slider controls whether the generated messages are added to the Recorded phrase. The default value of this button is NoRec, so that the controller messages are not recorded. Changing the value to Rec (by pressing the button and selecting "Rec") will cause all of the controller message to be recorded. This would mean, for example, that you could play back something in the Group tool, adjust the volume in realtime by moving the slider of the Volume tool, and then use the File->Add Track->Last Overdub feature of the Group tool to add the volume controller messages as a new track.