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.

^ This is my understanding as well.

Most people save scenes as part of a WIP, so of course they might make changes to specific figure as they go along, then render or export for postwork.  For me, my example above is the completed piece when I decide there are no more changes to make in the stock/default scene, so it has always been how I save a multi-figure pose set since I always intend to adjust the figures/props to make them specific characters that are already positioned, lit, and shot in a stock setting.  Since all of my poses have the figures dropped to the floor (regardless of the actual pose), I want them to load on the ground and not floating.  If I ever make Supergirl or some such, I will pay attention so the fly/hover poses save unattached to the ground just so I do not have to fuss with detaching them later.