In Daz Studio, a shell is a mesh with zero vertices that is used to add features to the materials of the base mesh. It can be used e.g. for makeup, tattoos and wounds. Shells are also used by Genesis 8 genitals to add color and effects without having to modify the torso material.
Importing the shells directly into Blender makes no sense, because they are empty meshes that don't show up in renders. Instead the Daz Importer uses the information in the shell materials to modify the materials of the base mesh. Here is an example where two bruises have been added to the face and one bruise has been added to the lips:
A different way to add layers is to use the Layered Image Editor (LIE) in Daz Studio. Here we have added three small textures to the face texture.
After importing to Blender, the face looks like this. There was also added an extra texture to the lips material.