Overpass API: map_to_area

The map_to_area maps OSM relation IDs to their corresponding (pseudo) area id, i. e. the function adds 3600000000 to the id.

relation[name=Pfungen];
map_to_area;
out;

As per a note found in this documentation, the main purpose of map_to_area is to search for objects inside an area, which is again inside another area ("area in area query").