Primitive type 'xs:ID'
ID represents the ID attribute type from [XML 1.0 (Second Edition)]. The "value space" of ID is the set of all strings that "match" the NCName production in [Namespaces in XML]. The "lexical space" of ID is the set of all strings that "match" the NCName production in [Namespaces in XML].
Any combination of letters, numerals, "-" and ".", with a length limit of 64 characters. (This might be an integer, an unprefixed OID, UUID or any other identifier pattern that meets these constraints.) Ids are case-insensitive.
All FHIR elements must have a @value or children
hasValue() or (children().count() > id.count())
unique id for the element within a resource (for internal references)
Must have either extensions or value[x], not both