New to GPSMAPEDIT

[popej]

NT is a native Garmin format, which is not revers engineered yet. As far as I know, there are no 3-rd party tools which can decode this format. You would need Garmin MPC compiler to create map in this format:
http://garmin.blogs.com/mpc/map-product-creator.html

Quasi-NT is a common Garmin map saved in a structure like NT map. I have introduced this format in GMapTool and it is supported by cGPSMapper. It can be decoded by knowledgeable person, but tools are difficult to find.

You can do some additional damages to map file, which are ignored by Garmin GPS but not by decompiling tools. This can give you some additional protection against casual hackers.

[Boyd]

You will find some info here: http://pinns.co.uk/osm/RGN.html