A commonly used term to describe the ability of a control valve or regulator to shut off completely against any pressure on any fluid. Unfortunately it is completely unrealistic. Control valves are tested to ANSI B16.104 and FCI70-2- 1976 which is the American National Standard for Control Valve Seat Leakage. This standard uses 6 different classifications to describe the valves seat leakage capabilities.
The most stringent of the these is Class VI which allows a number of bubbles per minute leakage, depending on the port size of the valve. The correct response to the question "Will that valve go “Bubble Tight”? is to say this valve is tested to meet Class VI shutoff requirements.