Users like to keep the Stack as clean as possible, and currently to filter specific parts of a mesh for the UVW Xform modifier requires Vol. Select modifier, or modifiers that modifies the Stack selection in some way. This can grow the Stack unnecessarily large.
A suggestion to make the Stack a bit cleaner is to have the UVW XForm modifier include a MaterialID filter so no extra modifier is needed to filter MaterialID polygons, which is a common need.
To turn off filtering by MaterialID the spinner could be set to 0, or by unchecking an adjacent checkbox.
Mockup of MatID paramter