the list-order of terrains
mattersbecause terrains are /not/ contained in .MAP files ... what happens when there is more than one terrain, is that they all get scrunched together into (what can be considered as) one bigger terrainset -- at runtime.
If, for example, in mv2 you mouseover or select a tilepart in TileView, both the titlebar and statusbar will show "terId" and "setId" ... terId is the 0-based id of a part in its terrain, setId is the 0-based id of a part in the map's entire terrainset. if you want a wtf-moment try switching the order of allocated terrains in the TilesetEditor ...
a Mapfile contains only the setIds [and the x/y/z dimensions]
So, a well-formed block has a .MAP, .RMP, plus
a ruleset defining its terrains [as well as other things].