Well this is a really late reply, but I thought I'd answer it anyways.
The different maps are for different areas and facets in UO. The main difference is the size of each map.
Map0 = Britannia (Felucca)
Map1 = Britannia (Trammel)
Map2 = Ilshenar
Map3 = Malas
Map4 = Tokuno
Moving from one map to another is handled server-side, and scripting that depends on your server emulator you are using.
There are a few utilities for converting Map to Bmp but they are mostly not complete and/or not fully capable of doing a complete conversion.
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