The first three paired elements correspond to ATA chapter, section and
subject respectively.
On the top level are ATA chapters, which deal with the different aircraft
assemblies and systems.
Such as chapter 79-Oil system.
The second number element refers to sections within chapters, e.g.,
79-21 stands for Oil cooling.
The third number element refers to subjects within sections, i.e. to
separate components; e.g., 79 -21-51 stands for one of the coolers, the Air-
oil heat exchanger.