Close but slightly different. When you aim the camera at the subject it starts looking for focus and before focus locks and the AF point turns green, it may move the position if the detected face moves. In effect it is tracking the location of the face before you lock the AF. With face select OFF, then the tracking doesn't move the AF point with the face, it stays where the face was initially found.
You can see this for yourself even on a single face. Aim the camera at the face and press the shutter to initiate focus if the AF point turns green release the shutter button and now if you move the camera the af point moves to track the face that had been found, so that when you press the shutter again the AF point is already on the face even if it has moved. With face select OFF this tracking of the face initially found doesn't happen.
In my own experience I actually stopped using one shot AF with my EOS R6 for portraits and simply used servo AF., and there is no setting for face select OFF in servo. Servo AF will still find the face or eye and tracks it around the frame if the subject moves, while also adjusting focus if you or the subject change the distance between you even slightly. For me this give me more accurate AF even with very fast lenses.
Brian
EOS specialist trainer, photographer and author
-- Note: my spell checker is set for EN-GB, not EN-US --
