Centered

Window appears centered on the visible area of screen.

Moused

Window appears centered under the mouse pointer.

Default for this tag is Centered.

As long as your window has a window id (Window.MuiID), choosing a position is not that important. MUI will always remember a windows last position and size and these values will simply override your settings. Positioning and sizing should be completely under user control, a programmer doesn't need to worry about it.