ATA Chapters

ATA chapters (sometimes called "ATA 100 System Codes") are a way of categorizing the various systems that are on an aircraft, originally created by the Air Transport Association in 1956. ATA chapters are configured like this: 12-34-56. This most important numbers are the first two. So in this example here, you would say, "That's chapter twelve."

To continue, see the Wikipedia link that tells the ATA story nicely: https://en.wikipedia.org/wiki/ATA_100 The Wikipedia link lists every ATA chapter in deep, deep detail. This is more information than you will ever need.