The outflow valves are designed to give a controlled pressurization, the relief valves or if part of the same unit will operate at a higher differential to prevent structural damage and likely move rapidly. In the older models testing we would cover their outlet with a caste chamber and evacuate that, to cause the differential, when the valve 'popped' open the units gauge would show the pressure at which it activated; in olden days it was often a problem that nicotine build up would delay this action. Negative relief valves are usually a separate item, very often fitted in the rear pressure bulkhead to allow air into the fuselage in the event of a sudden aircraft decent before the pressurization supply could catch up.