This dialog allows you to change the properties of the selected mask area. 

Shown on

Mark one or more elements from the list of courses. When you mark "All courses", the Mask Area object will be shown on all courses, including those courses that are not marked in the list

Mark one or more elements from the list of canvases. When you mark "All canvases", the Mask Area object will be shown on all canvases, including those canvases that are not marked in the list.

Fill color

Click on the color box to select the color of the mask area.

Drag the slider to configure transparency of the mask area.  The area can be completely Transparent, where the mask area is invisible, all the way to Opaque, where the mask is solid. Keep in mind that when printing on a PostScript based printer, transparency will be disabled.