Select Disable to disable the a button, preventing live view from starting accidentally. If Enable (standby timer active) is selected, the a button can only be used to start live view while the standby timer is active.

A Custom Settings: Fine-Tuning Camera Settings

a: Autofocus

b: Metering/exposure

c: Timers/AE lock

d: Shooting/display

e: Bracketing/flash

f: Controls

g: Movie