Do you remember the movie the Karate Kid? The famous Mr Miyagi said:
Wax ON, Wax OFF!
— Mr Myiagi
Canon gave us the AF-ON, AF-OFF. What’s up with the autofocus off? The AF-OFF? What’s the purpose of the autofocus off? You can assign a button to the autofocus off with the Customization C.FN IV - 1.
The default of is that the autofocus OFF is on the lens but…
What’s the purpose of AutoFocus-OFF?
It’s all about controlling the focusing system.
When the Canon 7D is set as a One-Shot AF, you either half-press on the shutter or on the AF-ON button at the back, and you are done. The focus has been achieved and there’s no more autofocus operation.
When the Canon 7D is set as AI Servo AF, its purpose is to track a moving subject. We know when to start, that’s when you either half-press on the shutter or on the AF-ON button at the back. The problem is when does the 7D stops tracking?
If you have remove the AF from the shutter butter and only use the AF-ON back button, then whenever you stop pressing on the AF-ON button the tracking stops.
If you have kept the AF on the shutter button, you can indicate to the 7D when to stop tracking with the AF-OFF.
To access the AF-OFF, either use the lens, but it’s far too difficult to remove the hand from the camera, find the switch on the lens and place the hand back on the camera. It takes forever and very likely will cause some problem or some mistake.
Or you can assign the AF-OFF to one button at the back with the Menu → Custom Functions → C.Fn IV: Operation/Others → 1: Custom Controls
Or you can remove the focus from the shutter button and use the AF-ON button. When my thumb is on the AF-ON, the tracking applies. When my thumb is off the AF-ON button, the tracking stops.