 model can be queried via discovery protocols (TAP specifically), 
but the normal use would be to use specialised services that make use of this metadata to 
orchestrate the data access. Specifically, Artifact-Part-Chunk metadata is designed to support
the implementation of IVOA DataLink and SODA services. The Plane-Artifact relation maps
directly to the one or more download links provided by the standard DataLink output; you invoke 
DataLink with a plane identifier and get back URLs for all the artifacts. 