https://services.dnb.de/fize-service/gvr/html-service.htm
I spotted this service at the overview for the upcoming "Datenbezieher"-Workshop of the DNB:
https://wiki.dnb.de/spaces/ILTIS/pages/125420735/Datenbezieher-Workshops
JOP provides an overview if an institution has phyiscal holdings or online access to a resource.