The idea is to have a civilian legal carbine that mimics the Army's current issue carbine as closely as possible. You can not easily or cheaply have the exact carbine for a couple reasons: The Army's M4 is select fire, the Army's M4 has a 14.5" barrel, if you are using a post-ban lower reciever you also can't have the flash suppressor or the collapsable stock.
So, you end up buying an upper reciever with a 16" barrel: 16" including a muzzle brake made to look like the military issue flash suppressor. The barrel will have a portion just in front of the front sight that is smaller diameter than the rest of the barrel; on the real M4 this cut-out is for mounting an M203 grenade launcher. On yours, it is for looks. You get either a fake collapsable stock or buy a pre-ban reciever so you can have the real thing. Oh, and you want a flat top upper reciever.
So, we call this a forgery or M4orgery since it is not the real thing, but is as close as we can easily get without jumping through a whole lotta hoops or breaking any laws. In the end you have something that looks very close to the real thing.