Nikon D6 Reference Manual page 221

A
Choosing Custom Focus Groups
The dimensions of the custom focus groups for
[Group-area AF (C1)] and [Group-area AF
(C2)] can also be selected using Custom
Setting a10 [Custom groupings (C1/C2)].
Select the desired custom group AF-area mode
and size the group using the multi selector.
Press J to save changes.
169
Choosing How the Camera Focuses (Focus)