Air transportation Association (ATA) published a numbering system to learn and understand the technical features of an Aircraft. ATA chapters make it easy to categorize technical information. Looking at any Component Maintenance Manual (CMM) for any civilian aircraft. At the bottom of every single page is the ATA chapter for that manual. The unique aspect of the chapter numbers is its relevance for all aircraft. Thus a chapter reference number for a Boeing 747 will be the same for other Boeing aircraft, a BAe 125 and Airbus Aircraft. Examples of this include Oxygen (Chapter 35), Electrical Power (Chapter 24) and Doors (Chapter 52).