When does a firearms reciever or suppressor need to be marked?

I have been told I had to serialize and register to 12' lengths of DOM tubbing as suppressors as soon as it came in the door. I have also been told that it does not have to be marked and registered until it is to the point that anyone can screw parts on and have it function as well as it doesn't have to be marked until it goes out the door.

I understand that there is a big difference between when I am making 1, 2, or 6 suppressors in a run and Gemtech making 500 in a run, but we are both manufacturers and there is no difference at what point the tube becomes a suppressor and they should be marked. I just would like a simple answer for a simple question from ATF.

Again everyone is playing buy different rules. To me this is pretty simple stuff as to when something becomes a usable item of course I am just a gunsmith, what do I know about guns.