As you know, units typically stand on the roadWay LOD, and if it doesn't exist, they stand on the GEOM LOD. So a command like this:
lineIntersectsSurfaces [top, bottom, objNull, objNull, true, 1, "ROADWAY", "GEOM"]
can give the next surface where you can stand on.
It is very useful when you want to place something on top of another object.