Mask Position is the next option, which positions the mask over the image, and allows you to display different areas of the image. Here you have a choice of Top Center, Top Left, Top Right, Center Center, Center Left, Center Right, Bottom Center, Bottom Left, Bottom Right, and Custom.
If you choose Custom, a new option appears called Custom Mask Position, where you enter values in pairs, first for the X axis, and secondly for the Y axis, as coordinates for the position ex. 60% 50px.
Below from left to right, we have Center Center, Center Left, and Center Right. Note how different areas of the image are showing through the mask.