from the .map file, when the client reads it, instead of saying it looks like this, the .map fiel pretty much tells the client that say at cords 21,55, there is image 1192 from Object2.wil
thats how it is done, when u load into a map pretty muc hthe map file tells the client what objects/tiles are on the map so the client can load it. but the server also has to tell the client as well, so the client comapres the server map to the client map, and if there are objects/tiles missing from the client map thats on the server map, it turns to black...
a bit confusing the way i explained it :P