What is omit-xml-declaration in serialization options?

What is omit-xml-declaration in serialization options?

XML Output Method:

The omit-xml-declaration and standalone Parameters
The XML output method MUST output an XML declaration if the omit-xml-declaration parameter has the value no.

The XML declaration MUST include both version information and an encoding declaration. If the standalone parameter has the value yes or the value no, the XML declaration MUST include a standalone document declaration with the same value as the value of the standalone parameter. If the standalone parameter has the value omit, the XML declaration MUST NOT include a standalone document declaration; this ensures that it is both an XML declaration (allowed at the beginning of a document entity) and a text declaration (allowed at the beginning of an external general parsed entity).

A serialization error [err:SEPM0009] results if the omit-xml-declaration parameter has the value yes, and the standalone parameter has a value other than omit; or
the version parameter has a value other than 1.0 and the doctype-system parameter is specified.
The serializer MUST signal the error.

Otherwise, if the omit-xml-declaration parameter has the value yes, the XML output method MUST NOT output an XML declaration.