This text file, an output from bigmap, lists all of the maplets that were used by bigmap the last time it was run. This list includes maplets contained completely by the bigmap as well as those that fall on the border.
USED_MAPS.TXT is different from INSIDE.TXT, which only lists maplets contained entirely inside the bigmap that was created.
I tweaked above wording slightly. Check to make sure it's accurate.
Here is a sample USED_MAPS.TXT file:
T00144 T00145 T00146 T00160 T00161 T00162 T00163 END
(Compiled by TC)