Prop class generates the well known proportional gadgets. It offers the same attributes as a usual boopsi gadget of propgclass. However, MUI's prop gadgets allow using any imagery for the knob and for the background.
Note that this class won't create any navigation buttons for the scrollbar. If you'd like to have two navigation buttons to be connected with the scrollbar, use Scrollbar class instead. See Scrollbar class for details.