If you are able to initiate focus with the shutter button, you have not set custom setting #A4 to "AF-ON only". In that case if one presses the AF-ON button to initiate focus, releases the button, then presses the shutter button, the press of the shutter button will refocus the camera. To keep the camera from refocusing when the shutter button is pressed, one must set custom setting #A4 to "AF-ON only". If that is done, then my understanding of how it works is:
1. Single press (then release) of AF-ON button will lock focus. One can then recompose and press the shutter button; focus will remain locked on the chosen subject.
2. Keep AF-ON pressed. The camera will track focus as long as one keeps the AF-ON button pressed. One can press the shutter button as many times as desired or, if CL or CH frame advance is selected, continue to fire away and the camera will continue to track focus.
I do not know of any purpose served by using the AF-ON button unless custom setting #A4 is set to "AF-ON only". If any one uses the button in that way please point out what purpose is served by doing so. I have not been using the AF-ON button very long and am still learning, so will welcome any advice any one has to offer. Any comments and criticisms of the above is also welcome.