The X,Y,Zres sliders have been a part of ZBrush I have never payed much attention to, but I finally got around to checking them out.
These sliders can be found in the Transform palette, although as they affect only adaptive skining I wonder if they might be better put in the Tools>Inventory palette.
The Xres, Yyres and Zres sliders control the resolution of zspheres, specifically those that are adaptive skinned. A full explanation of them can be found in the manual in the Transform palette section. In addition to that, if you use Digits Widget (found in the quicklinks) for making zsphere models, this enables you to change the resolution of the selected zsphere in any axis even after it has been drawn.
To get a better understanding of what they do I put together a simple zsphere tool and played with the various combinations; the results are shown in the picture below. The numbers refer to the values entered for the Xres,Yres,Zres sliders in that order and were applied only to the top zsphere. What they effectively do is make it possible to increase the number of edge loops in a particular direction.
When all sliders are increased to the same amount this replicates what the Ires slider in the Tools>Inventory menu does, except that the Ires slider applies the change to all zspheres in a model that have over a certain number of other zspheres attached to them. Using the X,Y,Z sliders you can apply changes selectively to single zspheres.
The thing that spurred me into looking at zsphere resolution was being unhappy at the way I have been doing hands.
My usual style of hand is shown in the picture below. The problem with it is that the edge loop that runs around the base of the fingers also joins the thumb, which makes it really hard and a long job to push vertices into the right place.
I figured that if I could increase the resolution of the 'palm' zsphere where the fingers joined it I would be able to add more zspheres to that side without making a messy mesh and to 'separate' the thumb from the fingers.
After experimenting I found the optimal resolution for the hand I was working on was 0,5,8 (EDIT CORRECTION: I think the resolution was in fact 0,7,5 - damn my memory
). It may look counterintuitive, but no tweaking was done to the mesh before the first skin. It gave nice edge loops, with plenty running from the wrist to the fingers so that details like tendons could be put in. It looks rubbish from the side though, but a quick flattening and smooth fixed that in much less time than it would take to push vertices around.
Here's the hand tool (XYZ res_ZSphere hand.zip) if you want to have a play. If you want to pose it, you'll probably need to add more zspheres in the fingers to help define the knuckles.
I think these sliders could be a really powerful tool when making zsphere models and is certainly something that I have been underutilising. It was great to learn something new too