I'm not sure exactly what you're referring to here but any device which threads to the end of a gun and is designed to or actually does reduce it's sound levels is defined by the ATF as a silencer and requires specific paperwork to be legal.
There is no legal (or other) distinction between "silencers" and "suppressors". They are two names for identical devices.
"FLASH suppressor" is what I meant by suppressor. A screw-in hollow canister type, it's just an empty metal housing, no sound deadening materials involved. it's designed to suppress muzzle flash by providing a hollow contained area to burn excess gunpowder exiting the muzzle. I think in some cases they are sold as a "fake silencer" to achieve a certain weapon look without actually having a silencer. Does not require a class 3 permit, and is legal for most people in most states.
It's not "designed" to reduce the sound levels, and it's no silencer by any stretch of the imagination. But it does take the edge off of the gunfire report 'somewhat'. Much the same as a muzzle brake is not designed to make guns louder, but does anyway.