Head Detect AF: This is totally new for Canon — and likewise enabled if “AF Priority (People)” is activated. The idea here is to be able to identify not simply faces, but human heads. This is especially powerful when a full face can’t be identified (such as a face partially obscured by a visor on a hat).
And, if Face Detection is initially active, but a subject’s face becomes unrecognizable for any reason (sudden strong back-light, subject turns away from camera, etc.), the EOS-1D X Mark III’s AF system can seamlessly switch from Face Detect AF to Head Detect AF, and maintain recognition of the primary subject.