There are three types of SOTs, importer (Class 1), manufacturer (Class 2), and dealer (Class 3). Any SOT can deal in NFA weapons so they are all NFA dealers.
A Class 1 SOT is an importer but may also deal in NFA weapons.
A Class 2 SOT is a manufacturer of NFA weapons, can also deal in them, and has very limited importation privileges.
A Class 3 SOT is a dealer and can only deal. They cannot import or manufacture.