Reference: *Structure Module at the W3C
The Structure Module declares the elements used in the basic structure of an XHTML 1.1 document.
For example, the code below shows how these elements are used in a typical XHTML 1.1 document:
The first two lines of code in the above example are not part of the Structure Module but are used to inform the document's parser that the document is XML and that the particular flavour of XML being used is XHTML 1.1. For more information, see XML Declaration & DOCTYPE.