緊急用 also means 'for emergency use', but 緊急 emphasizes urgency, while 非常 is broader.
常用 refers to items or medicine used regularly in daily life, while 非常用 refers to items reserved specifically for emergencies, not everyday use.
emergency power supply; backup power source
contingency plan; emergency countermeasure plan
emergency eject hole
Compound of 非常 (emergency) + 用 (use, purpose).