Assuming a document system involving multiple authors each with their own documents:
A user should be able to select text in another document and use it as a quotation in their own document.
This quotation should be a link which will take a user back to the referenced text.
This quotation should link to the particular version of the document that is referenced.
Users should be able to query a document to show a list of other documents which reference it.
Users should be able to query a document to see which parts are most often quoted.