The length of the number can reach 128 characters.
Publishers can assign the DOI to any level of work or product.
Attaching a DOI to the whole does not preclude also assigning DOIs to the parts. For example, a book can carry a number, as can a chapter, illustration or table. The directory works, in effect, as a router. Holding all the DOI numbers and the address of the server maintained by the publisher, it serves as an intermediary between the user and the rights-holder. When a publisher moves, changes servers, or sells rights, the new location is updated in the directory, but the number remains the same, always attached to the same content.