A and B checks are lighter checks, while C and D are considered heavier checks. Aircraft operators may perform some work at their own facilities, but often checks, and especially the heavier checks, take place at maintenance, repair and overhaul (MRO) company sites.

In first aid and emergency medicine, ABCD checks are the fundamental steps of the "Primary Survey" used to assess a person who is unconscious or severely ill. A stands for Airway: ensure the person's mouth and throat are clear of obstructions. B stands for Breathing: check if the person is breathing normally by looking, listening, and feeling for air. C stands for Circulation/Compressions: check for signs of severe bleeding and, if the person is not breathing, begin chest compressions (CPR). D stands for Defibrillation/Danger: in some protocols, D reminds you to check for Danger to yourself first, or to use an Automated External Defibrillator (AED) as soon as it is available. These steps are designed to be performed rapidly and in order, as they address the most immediate threats to life, ensuring that oxygen continues to reach the brain until professional medical help can take over.