Canon EOS 5D Mark III Instruction Manual (en)

Function settings particular to Live View 
shooting are explained here. The menu 
options under the [z4] tab are 
explained on pages 205-208. In the 
A> mode, it is displayed under [z2].
The functions settable on this menu 
screen only apply during Live View 
shooting. These functions do not take 
effect during viewfinder shooting.
Live View shooting
You can set Live View shooting to [Enable] or [Disable].
AF mode
You can select [Live mode] (p.209), [u Live mode] (p.210), or 
[Quick mode] (p.214).
Grid display
By displaying a [3x3 l] or [6x4 m] grid, you can check for any 
picture tilting. Also, with [3x3+diag n], the grid is displayed 
together with diagonal lines to help you align the intersections over 
the subject for better balance in the composition.
 Menu Function Settings