Using FrameMaker, you can import and export structured documents in either SGML or XML (including XHTML 1.0) format. Once you import a structured file, it is no longer an SGML or XML file; it is a structured FrameMaker document. To return it to its original format, save it as an SGML or XML file.
SGML
Standard Generalized Markup Language (SGML) is the international standard for all markup languages for data exchange and storage.
SGML is a descriptive, rather than procedural, markup language, meaning the same document can be processed by different systems. Each system applies different processing instructions to relevant sections. You can transfer SGML documents from one system (hardware and software environment) to another without any loss of data.
SGML was the first language to implement the Document Type Definition (DTD), which formally defines the document by its components and structure. Documents of the same type can then be verified and processed uniformly.
A document that conforms to the structure of a DTD is said to be valid.
XML
Extensible Markup Language (XML) is a generalized format for representing structured information, especially for the web. Like HTML and SGML, XML requires the use of elements and structure.
However XML differs from HTML in that it is extensible. You can define not only your tags but also their order, relationships among them, and the way they are processed and displayed. In terms of markup, XML has tags or elements which are similar to HTML markup except that they are defined by you.
Use XML to define and implement a structure that is appropriate for your content. An XML document that conforms to the structure of a DTD is said to be valid. An XML document that uses tags that conform to the standard XML specifications is said to be well-formed.
XHTML 1.0
Extensible Hypertext Markup Language (XHTML) is an extension of HTML that is based on XML and is designed to work with XML-based applications. It can be viewed, edited, and validated with standard XML tools. Using XHTML is an easy way to migrate from HTML to XML while retaining forward and backward compatibility of your content.
XML vs XHTML 1.0
Whereas HTML describes formatting, XML describes content itself. Humans can read HTML documents rendered in a browser. Both machines and humans can read XML.
Instead of style-based, paragraph-oriented word processing and desktop publishing, XML provides a foundation for structured authoring. XML describes content according to elements that are organized in a hierarchical tree. In word-processing environments (such as unstructured FrameMaker), the relationship among the various document components is apparent through formatting on the page. The document file, however, does not capture these relationships because a word processor document is made up of a string of paragraphs. For example, unstructured FrameMaker does not capture the subordination of a Body paragraph tag to its preceding Heading1 tag. Structured authoring, however, does capture the hierarchical relationships among the document components.