Functional requirements for bibliographic records (FRBR) is a conceptual model of the bibliographic universe to describe entities, relationships, and attributes. The first group of entities are Work, Expression, Manifestation, and Item (WEMI). A Work is intellectual or artistic creativity and it is realized in an Expression, which is embodied in a Manifestation and exemplified in an Item. This is easiest to wrap your head around with something like Hamlet. The story was created in the brain of William Shakespeare (or whoever). It was realized in something like The tragicall historie of Hamlet Prince of Denmarke which was embodied in an edition printed in 1603 in London for N.L. and John Trundell and exemplified in the copy at the Huntington Library. Lots of print resources only come out in one edition and this is a heck of a lot of infrastructure for the library catalog to carry.
In the library cataloging rules such as Resource description and access, relationships and attributes are linked to particular levels. For example, the creator William Shakespeare is a relationship to the Work. If the Huntington copy has a later binding, this is a relationship to the Item. This all gets more complex for music and visual arts and other resources. There's a concise and helpful discussion of "FRBR, WEMI & music" on the website of Gilmore Library of Music at Yale.
Even more concise is a quotation from Michael Tilson Thomas in an article in last Sunday's Times. The quotation is from a 2017 interview. "I have encouraged people to color outside the lines for lack of a better analogy. We're not trying to reproduce the notation here. We're trying to get back to the inspiration that caused the notation to exist." It is not often that FRBR and WEMI are spoken of so aesthetically.