If you have loaded several maps covering the same area and these map overlap totally or partially, you can establish a certain viewing order. The map situated at the top of the list will overlay all the other maps. The next on the list will have preference on all the other maps except the first map, and so on.
To change the position of a map on the list, mark it (left click on it) and while you hold down the ‘Alt’ key, click on the arrow pointing up to move the map up a position, or click on the arrow pointing down to move the map a position down.