NT means New Technology and it's a Garmin compression technology for routable Garmin maps. Using NT format for a non-routable map actually makes it larger and it's NOT recommended to build this option according to Garmin.
NT maps was first introduced when memory storage and memory cards was very expensive. In 2004/2005 you would have to pay ~700 USD for a 4 GB memory card.
Garmin uses NT format to protect maps from editing and stealing details by other map makers. OF / non-NT (old format / legacy format) maps can be edited by some map editor tools. The map data is normally the same in the NT & non-NT maps but some details might be removed in non-NT maps if it's close to 2 GiB in size. Older units have a 2 GiB limit of file size for map files.