Auto Groups will create a new group for each separate poly object. Keep in mind that a separate poly object is not a SubTool but a part of the object that does not share any polygon faces and is self-contained.
The Group Visible button will create a single Polygroup from the visible polygons. If the Coverage and Clstr sliders are used then random variations can be created.
The Coverage slider will apply Polygroup variations when the Group Visible option is used. A setting of 1 (the default) means that there will be complete coverage and only one Polygroup will be created.
The Cluster slider controls how Polygroups are in generally the same region when the Group Visible function is modulated by the Coverage slider.
This slider works in unison with the Coverage slider, causing the randomly grouped polygons to be more clustered or more spread out across the region. A high value results in the regrouped polygons being clumped together. With a low value, ZBrushCore will spread the regrouped polygons throughout the visible surface.
Group Masked Clear Mask
A Polygroup is created from the masked portion of the mesh and the mask will automatically be cleared.