From Seo Wiki - Search Engine Optimization and Programming Languages
In HTML and XML, a public identifier is meant to be universally unique within its application scope. It typically occurs in a Document Type Declaration.
A public identifier is meant to identify a document type that may span more than one application. A system identifier is meant for a document type that is used exclusively in one application.
In the following Document Type Declaration, the public identifier is
-//W3C//DTD XHTML 1.0 Transitional//EN:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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