... standalone="yes|no" ...

If a document is completely self contained (the DTD, if there is one, is contained completely within the original document), then the standalone="yes" declaration may be used. If this declaration is not given, the value no is assumed, and all external entities are read and parsed.

From the standpoint of an XML application developer, this flag has no effect on how a document is parsed. However, if it is given, it must be accurate. Setting standalone="yes" when a document does require DTD declarations that are not present in the main document file is a violation of XML validity rules.