one double-clicks (OS X) a material zone in the 3d layer screen, e.g. torso, then it opens a 2d layer.  by default in APSE, there is only one layer per material zone, but more can be added for blending purposes.  however, this blending operation can be done in the poser material room.  the latter is one of the FAQs here, usually in ref. to tattoos.  you may wish to ask the mods to move this thread to APS forum.