tnrk maps are build by the addtion of map layers. Each one of these map layers is a representation of the terrain in which the action developes, attending to an specific feature.
All map layers has a name, identifying its type. That name should have no more than 8 characters, due to restrictions in the implementation of the game datebase.
Some map layers are now supported, while other are just scheduled. Here is the list for the map layers.