An identifier, such as a DOI or archive control number, or LCCN for a series or set of books.
The @collection-id-type attribute should be used to name the type of identifier, such as “lccn”. There is a separate element for the ISBNs (<isbn>) related to a book.
<!ELEMENT collection-id (#PCDATA) >
Text, numbers, or special characters
BITS-book-part0.ent