After referring to my D850 manual(s) and the D6 manual(s) it appears that "Custom group area" is a feature of the D6 but not D850.
Right, this is correct. I'm hoping Nikon would include the D6 AF system in a D850 upgrade.
In single point AF I have set my D850 to switch automatically to the last point selected in horizontal or vertical orientation. When I press the AF Sub-selector (joy stick, center switch) my D850 switches from single point AF to Dynamic-area AF (9 points).
I use 9-point dynamic area quite a lot, but I tend to just use the focus area selector to switch between modes rather than programming it to a button. I find it tricky to use the joystick for AF-ON + AF Area Mode, I guess I am too clumsy to use it and accidentally move the focus point around. ;-) I think this is because I'm used to the thumb taking a part of the weight of the camera when shooting.
Face detection for the D850 appears to be limited to AF-Area Mode, 3D Tracking.
In the D850, in continuous AF, this is the case, although auto area AF may have some built-in sensitivity to faces (it could be better) and in single shot AF mode, group-area AF can also be used to detect faces and prioritise them. I never quite figured this out, but I am always using AF-C so I didn't investigate it further.
In the D6, one can choose to prioritise faces (or not) in 3D tracking, auto-area, and group-area modes and the face priority can be individually turned on or off. In fact for head and shoulders framings it'll focus on the eyes when this feature is activated, but for longer distances it selects the focus points for the whole face.
For the D850 I think my favorite modes are the 9-point dynamic, single point and in low-light I will sometimes use group-area as it seems to function in lower light than single point or dynamic area. For fast action I use 25-point dynamic for close subjects and group-area for distant subjects. But 9-point dynamic could be used for almost everything I shoot. For photographing people who are face towards the camera, I sometimes use 3D tracking but in my experience it sometimes loses the subject if briefly occluded (or turning the face away from the camera's view), so I don't have complete confidence in this mode.
In the D6, there is a bewildering array of options, and I've only figured my way around for some subjects so far. For figure skating I tend to use 11x3 or 7x3 matrix of points in custom group area mode (with face priority). For animals I think my default mode is 9-point dynamic, for small subjects between twigs, single point, for fast bird-in-flight, 25-point dynamic, though since the focus points are a bit bigger in the D6 than the D850, I use 9-point in some situations where I would use 25-point in the past. I sometimes use the custom group-area here also but without eye detection it's less useful for close animal subjects than it is for humans. For long-distance photos, group area (custom and regular) works well. Wildlife activity is greater in the spring and summer here and so I'll get more data on how it works for me in different modes. Though it might be that new movement restrictions greatly limit my possibilities.
