Hey guys many times during my exterior walkaround I noticed on the A320NEOs that they have this pressure relief door and our SOP for the exterior walkaround states to check that this door is closed. However I would like to know what is the purpose of this door? As it's not mentioned in the FCOM.
Inside the nacelle cavity of a turbofan engine and in the casing of the engine core, there are different zones or closed compartments used for ventilation/cooling and in the case of the nacelle lip, also for anti-icing (see the first two images below). If a highly-pressurised duct (such as the anti-ice inlet duct shown in the 3rd image) bursts, the pressure builds up to a dangerous level, despite the ventilation exits. To prevent structural damage to these closed compartments, pressure relief doors are installed which are triggered automatically.
Edit: my first answer lacked some details, so I rewrote it.