The SGML standard incorporates the concept of a DTD (a Document Type Definition).
The SGML DTD scheme is similar to, but more complex than the XML DTD mechanism, as it contains additional features. In some cases, these features can be emulated using other techniques.
An SGML DTD and its associated documents can be converted into equivalent XML files, should there be any reason to replace SGML tools with XML tools, or to process, deliver or publish SGML data using XML tools. But it may not always be possible to achieve this without human intervention.
A content model can simply include the keyword ANY, which indicates that any combination of text and the other elements defined in the DTD can be used in the content.