Now superseded by JASC (Joint Aircraft System/Component Code), ATA 100 was one of the specifications developed by the Air Transport Association of America.
It's a 3-part code used in Maintenance Manuals, Parts Catalogues and other technical documentation, of which the first part (Chapter) and usuallu the second part (Sub-Chapter) is common across most of the industry. So, for example, a mechanic pretty well anywhere will recognise that 32-40-nn is where to find information on Landing Gear -> Wheels & Brakes.