OAI-PMH supports a system for the creation of sub-collections that it calls "sets". oai4courts adds database support for a less-formal system of tags. Sets are named according to a hierarchical system that implies an equally hierarchical partitioning of the database. Tags may be applied in any way you like.
OAI-PMH repository implementations consist of request- and error-handling scripts that draw on a relational database (RDBMS) for the metadata they deliver. In practice, this is because implementations that relied on real-time extraction of metadata from documents in a file tree would be unacceptably slow. So for practical purposes the question becomes one of how the metadata finds its way into the RDBMS, and that in turn depends on the nature of the documents themselves,
[this is mostly a placeholder for now -- unedited notes follow]
Here's what we have in mind:
oai_lii:law.cornell.edu:us/federal/scotus/00-201-ZO-html
This example is the identifier for the majority opinion in NY Times v. Tasini.
It breaks into
scheme:namespace-identifier:local-identifier
-- the scheme oai_lii should be the same for all identifiers, everywhere, and
will eventually
be associated with a series of standards documents and formal xml schemas.
You will also want to look at the conceptual overview of Level 1 data.
Here's a schema diagram. It won't print well, sadly. There's a PDF version that might be more readable: OAI-lite.pdf
