If you have selected a frame which contains image data, this menu item is selectable. When you select it, a control pane is shown below the selected frame.
The control pane shows various elements which can be used to edit the mask and image itself:
When you select this left control segment, the image data object will be selected for editing.
When you select this right control segment, the mask object will be selected for editing.
Moving the slider adjusts the scale of the image.
Click this button to exit the Edit Mask mode. The control pane will be removed from screen. Alternatively, [Escape] also closes the control pane.
