OF stands for Old Format or Original Format. It is the Garmin map image file format preceding NT format. I have only seen OF used by Garmin China and Garmin Taiwan to distinguish maps in old format from the new NT format. See an example below. Here in Noeman, it is otherwise referred as Non-NT.
Originally Posted by jpooh
If there are not many changes in your area, you may want to find an old map of your area in OF format so that you can open and edit it, add new roads and POIs if necessary. By that you can have a corrected and updated map of your area.
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