Garmin can display multiple map at once. What you will see, which map will be on top, depends on 2 main factors:
- type of the map
- internal map priority and transparency settings
There is a list indicating priority according to map typ, from the top to bottom:
Water Waypoints and Lights
Road & Recreation
I'm not sure how precise is this list, but i can confirm, that type of the map is important.
Second factor is draw priority. It is a number in range 0-31 hidden inside img. This value can be edited by some tools. Map with bigger priority are displayed over other maps.
Map can have transparency attribute. It makes map polygons invisible. Transparent maps are displayed over non-transparent maps.
Both factors are used by GPS when drawing maps, but i really don't know, how exactly draw order is calculated. Draw priority is a simple number, changing it gives expect effect, but it is difficult to tell, how map type is converted or compared to priority numbers.