I expect you would want to assign a material group that matches your displacement map, which (I think!) would be tied to the UVMap.  The material grouping is independent of figure object grouping, as in body parts.  E.g. in a typical clothing figure, you might have Chest/Abdomen/Hip, whereas there will normally be a "Cloth" or somesuch group that covers all the geometry that material applies to.  I think the mapping of material groups ought to be done on the Zbrush side, since that's where you're making the displacement map.

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