In a Venturi-meter where the flow is always subsonic any change in either the pressure at entry or the pressure at exit from the meter, or both, will alter the flow through the meter and this is how the meter works.
For a convergent divergent nozzle with an overall pressure drop that is great enough for supersonic flow to occur throughout its divergence a change in the inlet pressure will alter the flow but the exit pressure can change quite significantly without affecting the flow. Providing that the flow in the divergence is wholly supersonic the flow can emerge from the nozzle either below the back-pressure or above the back-pressure. If this is the case there must be a pressure of the gas at exit that equals the back-pressure. At this pressure the jet will emerge with no net transverse pressure difference to make it either expand or contract. This is called the “correct” and sometimes it is convenient to use this concept.