Quote - I have noticed that the Select Subset is more importantly related to including the ground as part of the pose.  This selection anchors the pose to the ground; if it is deselected, the model will pose suspended in the air despite saving the pose back into the Library as on the ground.

Thanks - that's useful to know.

So, is the ground automatically selected if the "Select Subset" button isn't clicked on at all?

I did note that, as soon as I did click that "Select Subset" button, by default, everything in my scene (apart from the ground!) was in fact showing as selected, in the scene hierarchy view that pops up...