Rendering only: the terrain passes, the drape and the elevation fetches are dropped, but the tiles keep their terrain subdivision. Switching is instant, but a flat map carries 3D triangles.
"TERRAIN_FLATTEN_MODE_FULL"
The whole way: a flat map decodes, culls and draws as if no terrain were configured. The price is a re-decode at a switch, paid while the map is already flat; switching back at the same view reuses the last decode, kept in memory.
Rendering only: the terrain passes, the drape and the elevation fetches are dropped, but the tiles keep their terrain subdivision. Switching is instant, but a flat map carries 3D triangles.