Resource
A BibItem describes a bibliographical resource, or something similar. What does a Resource (record) describe?
We may want to add information about its embodiments (copies) to a BibItem. This is usually called Holdings data, and consists of strings, perhaps structured, detailing the institution, call number etc., with an URL as the limiting case!
What distinguishes Resources is that they are managed by or in cooperation with Pyblio. If needed, we could speak of managed resources.
To a BibItem more than one resource could be associated, and each resource may be composed of many files (objects). The ResourceServer stores those objects, if needed.