International Standard Book Number: a 13- or (if assigned prior to 2007) 10-digit identifying number assigned by the publishing industry to a published book or similar item, registered with the International ISBN Agency.
ISSN
International Standard Serial Number: an eight-digit number to uniquely identify a serial publication.
DOI
Digital Object Identifier: a unique string of letters and numbers assigned to an electronic document.
URI
Uniform Resource Identifier: a string of characters to uniquely identify a resource, following the syntax of RFC 3986.
VIAF
A data number in the Virtual Internet Authority File assigned to link different names in catalogs around the world for the same entity.
ESTC
English Short-Title Catalogue number: an identifying number assigned to a document in English printed in the British Isles or North America before 1801.
OCLC
OCLC control number (record number) for the union catalog record in WorldCat, a union catalog for member libraries in the Online Computer Library Center global cooperative.
<idno> should be used for labels which identify an object or concept in a formal cataloguing system such as a database or an RDF store, or in a distributed system such as the World Wide Web. Some suggested values for type on <idno> are ISBN, ISSN, DOI, and URI.
In the last case, the identifier includes a non-Unicode character which is defined elsewhere by means of a <glyph> or <char> element referenced here as #sym.