When a DTD or other external file is referenced from a document, the reference can be specified in three ways: using a public identifier, a system identifier, or both.

Public Identifiers: -//OASIS//DTD DocBook V3.1//EN

System Identifiers: /usr/local/sgml/docbook/3.1/docbook.dtd

In XML, System Identifiers must be URIs. such as file:///usr/local/sgml/docbook/3.1/docbook.dtd. or "http://nwalsh.com/docbook/xml/1.3/db3xml.dtd".

-- Comments are delimited by pairs of double-hyphens,
   as in SGML and XML comments. --
OVERRIDE YES
SGMLDECL "n:/share/sgml/docbook/3.1/docbook.dcl"
DOCTYPE  BOOK  n:/share/sgml/docbook/3.1/docbook.dtd
PUBLIC "-//OASIS//DTD DocBook V3.1//EN" 
  n:/share/sgml/docbook/3.1/docbook.dtd
SYSTEM "http://nwalsh.com/docbook/xml/1.3/db3xml.dtd"
  n:/share/sgml/Norman_Walsh/db3xml/db3xml.dtd