NATO STANAG 4774 (Confidentiality Metadata Labels) and STANAG 4778 (Metadata Binding) 4778-4.3.4: Encode embedded content and resolve references with the prescribed attributes and XPath rules

4.3.4 to 4.3.7: embedded Metadata and Data elements may carry xmime:contentType (default text/xml; charset=utf-8) and must carry an encoding attribute (for example base64Binary or hexBinary) whenever non-XML content is encoded to embed it. MetadataReference and DataReference must carry a URI conforming to RFC 3986 locating the metadata or data, may carry contentType (preferably an IANA media type; unregistered types are local only) and an encoding from the IANA transfer encodings, and may carry one ds:Transforms selecting part of an XML object, which must contain exactly one Transform with the XPath 1.0 algorithm URI, one or more XPath expressions and XML Signature XPath filtering semantics; local-name() and namespace-uri() are recommended to survive prefix changes.