Which of the following statements is true about ATA specification 100 publication contents?

• A. There are five levels
• B. Unit/Subject numbers are always 1-999
• C. The System Chapter is optional
• D. Unit/Subject numbers are not preassigned and may be selected by the Manufacturer

Answer: (D)

ATA 100 publication contents are organized around a flexible indexing system for units and subjects that the manufacturer defines. The standard provides the structure and cross-referencing methods, but it does not lock in fixed numeric assignments across all manuals. That means the actual unit and subject numbers aren’t preassigned by the specification; they can be chosen by the manufacturer to reflect how their systems and topics are laid out in their manual.

This is why the statement about unit/subject numbers not being preassigned and being selectable by the manufacturer is the correct one. It captures the practical approach: publishers choose the numbers to fit their content, allowing them to add new topics without conflicting with a global, rigid numbering scheme.

For context, you’ll typically see a manufacturer assign a unit number to a system and then a series of subject numbers for procedures, faults, adjustments, and troubleshooting within that system. The idea is to keep the manual coherent for that particular aircraft and its equipment, while still adhering to the overall ATA 100 framework. The other statements imply a fixed structure or numeric range that ATA 100 does not mandate, and the system chapter is not considered optional within the standard’s organization.