A DOI is a Digital Object Identifier. That is a Digital Identifier of an Object, not an Identifier of a Digital Object.
A DOI can be assigned to any Object. An object can be anything you want to manage: a piece of online content in a particular version, or a related object, such as an author or a party to an agreement; or a physical asset, a DVD or an item of equipment, for example.
DOIs can be assigned at different levels of granularity to suit differing needs, e.g. a collection, a book, a table within the book, the related source data for the table etc. When assigning DOIs, the registrant links metadata with an appropriate level of detail to identify a separate entity within the DOI system.
BiLD Law Journal has already applied for assignment of DOI. Soon each paper of the journal will be labelled a DOI.