These co-ordinates are floating-point numbers, and use the same relative co-ordinates as $Xpos and $Ypos. If a map view of a note is closed and later reopened, it will scroll to that map's most recently viewed position.
The scroll position is also reflected in the interior view of a note container viewport (as before an agent's contents are fixed). For a note container, data at $MapScrollX and $MapScrollY is drawn in the vertical/horizontal centre of the container viewport.
The viewport within a container can be moved via click-drag within the viewport area. Dragging the child map background scrolls the child map, dragging an icon moves that icon on its own map.