I just went ahead and stole the whole thing from Sebastian.

Seeing gun owners try to explain to other people the laws around machine guns is painful, especially when they get terminology wrong. Let us go over briefly some terms.The Federal Firearms Licensee

There are a lot of different types of FFLs. Let us review:

Type 1 – Dealer in Firearms (but not destructive devices)
Type 2 – Pawnbroker
Type 3 – Collector of Curious and Relics. I lot of you have this one, and so do I.
Type 6 – Ammo manufacturer
Type 7 – Firearms manufacturer (but not destructive devices)
Type 8 – Importer of firearms (but not destructive devices)
Type 9 – Dealer in destructive devices
Type 10 – Manufacturer of destructive devices.
Type 11 – Importer of destructive devices.

The Special Occupational Taxpayer (SOT)
You’ll often hear people refer to “Class 3 firearms” Ain’t no such thing as a class 3 firearm. Classes are Special Occupational Taxpayers. This is the license that allows an FFL to deal in NFA items.
Class 1 SOT – Importer of NFA firearms.
Class 2 SOT – Manufacturer and dealer of NFA firearms.
Class 3 SOT – Dealer of NFA firearms.

Title I and Title II
This refers to Title I and Title II of the Gun Control Act of 1968. This is how to categorize a firearm. Machine guns are Title II firearms. Rifles, shotguns and handguns are Title I firearms. Silencers are Title II items, but we’re trying to move them to Title I. The National Firearms Act regulates machine guns. It is part of the Internal Revenue Code. It was a 1986 Amendment to the Gun Control Act that banned any new machine guns from being registered to non-government entities, 18 U.S.C. 922(o). If you’re in possession of a Title I firearm illegally converted to a machine gun, you’ll be charged with violating 922(o), not violating the NFA. The government can’t prosecute you for failing to pay a tax it refuses to collect.

Source: Learn Your NFA Terms | Shall Not Be Questioned