Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 325)

Zone AF (manual selection of zone)
The AF area is divided into nine focusing zones for focusing.
Large Zone AF (manual selection of zone)
The AF area is divided into three focusing zones (left, center, and right) for focusing.
Automatic selection AF
The Area AF frame (entire AF area) is used to focus.
325