Adding a bit to what TSA said, you can also do stuff like
PHP Code:
addtiledef("tileset.png", "server_", 1);
addtiledef("cavetileset.png", "servercave_", 1); // or...
addtiledef("cavetileset.png", "server_cave-", 1);
for adding custom tiles for certain levels, rather than putting it in the actual level.