The starting UDIM number of 1001 is basically arbitrary. The last digit is the U (x) coordinate, which ranges from 1-9. So 1001, 1002, … 1009. The remainder of the digits are the V (y) coordinate. These are allowed to go above 9…once they do, they will ‘spill’ over into the first 2 digits. So 1001, 1011, 1021, … 1091, 1101, 1111, 1121, etc. If you needed a V coordinate above 99, then it would cause the initial 1 to become a 2. So 2001 would represent U=0, V=100.