Individual fragmented fragments each have a set of Vertex, Edge and Polygon Selection tags. As long as the Voronoi Fracture object has not been made editable you can define how the fragments should be displayed in the Object Manager (the tag name will be displayed there).
If you want to assign different materials to the fragment surfaces (Inside Faces) and to the surface (Outside Faces), use these tags to restrict the materials accordingly (this also works without previously making the Voronoi Fracture object editable).
These three options can be used to create different types of Vertex Maps. Selected points or edges will be assigned a weight of 100%. In conjunction with the Vertex Map shader, soft transitions, for example, can be created between different matierals from outer to inner surfaces.