There is only one file type that uses the .UNL extension:
Garmin Unlock File
Backup file created by MapSource, a program used to manage maps for Garmin navigational devices; contains unlock codes for maps; used as a backup copy so that the codes can be restored back to the Garmin device.
To create a UNL file in MapSource, select, click "Next," and choose "Backup my unlock codes to a file." To reload unlock codes, select , click "Next," select "Retrieve and reinstall all my unlock codes, click "Next," and then select "Retrieve my unlock codes from a file on my PC."
NOTE: Unlock codes are specific to a single GPS unit purchased from Garmin. Therefore, it is not possible to use a UNL file to unlock maps on multiple Garmin devices.
Updated: July 15, 2010