Quoted from a Ballebar post
the "gmapsupp.unl" file can hold all unlock codes in a Garmin GPS unit. If one needs 100 unlock codes for some reason then all can be put inside "gmapsupp.unl".
\Garmin\gmapsupp.unl" file as a consecutive string without delimiters or carriage return / line feed. The "gmapsupp.unl" file should be a multiple of 25 bytes (25, 50, 75 ... etc.).