A system identifier is a document processing construct introduced in the HyTime markup language as a supplement to SGML. It was subsequently incorporated into the HTML and XML markup languages.
In HyTime, there are two kinds of system identifier: formal system identifier (FSI) and simple system identifier.
In HTML and XML, a system identifier is a fragmentless URI reference. It typically occurs in a Document Type Declaration. In this context, it is intended to identify a document type which is used exclusively in one application, whereas a public identifier is meant to identify a document type that may span more than one application.
In the following example, the system identifier is the text contained within quotes:
Famous quotes containing the word system:
“In nothing was slavery so savage and relentless as in its attempted destruction of the family instincts of the Negro race in America. Individuals, not families; shelters, not homes; herding, not marriages, were the cardinal sins in that system of horrors.”
—Fannie Barrier Williams (1855–1944)