NIT check digit
The digit DIAN puts after a NIT, worked out from the number itself.
How this works
Each digit of the NIT, read from the right, is multiplied by a fixed weight. The results are added up and divided by eleven: a remainder under two is the check digit itself, and any other remainder is taken away from eleven. It is the check DIAN publishes, so the answer is the one printed on the RUT.
Paste the whole identifier if you like. Everything from the dash onwards is dropped before the sum, so 900.123.456-7 is read as 900123456 rather than as a longer number.