As far as I know, it depends on what you are building.  If you have a stripped post-ban reciever and are buying a preban parts kit you could get into trouble.   Other kits have barrel lengths shorter than 16" and can only be used to "repair" an SBR or registered machine gun.  I am sure some other factors apply but this should be the basics of it.

That warning often appears on parts kits from open-bolt guns or guns that can only be built as full-autos.  Only type 7 FFS with a Class II SOT can legally build those into guns, and then only for sale to other SOTs or the government.

Remember, it is illegal to possess parts that are exclusively designed for use in machineguns, unless you legally own that type of machine gun, where you can legally own most replacement parts.

-Troy