Areag.ini

From NWN1 Custom Content Guide

areag.ini is a file read only by the toolset, which sets how a newly created area should look like : it defines default terrain, fog and lighting. That is how when you create areas for some custom made tileset, you'll find it encaged in edges of rural forest tiles, or when you create a cavern one you end up with a single central room and you have to dig corridors. In theory someone could write areag.ini so that area is a large lava room, or that cliffs go around all the forests.