If the resource has been anonymized, this field can be used for entering more information, e.g., tool or method used for the anonymization, by whom it has been performed, whether there was any check of the results , etc.
Attempting to install the relevant ISO 2- and 3-letter codes as the enumerated possible values is probably never going to be a realistic possibility. See RFC 3066 at http://www.ietf.org/rfc/rfc3066.txt and the IANA registry at http://www.iana.org/assignments/lang-tag-apps.htm for further information. The union allows for the 'un-declaration' of xml:lang with the empty string.
Source
<xs:element name="anonymizationDetails"><xs:annotation><xs:documentation xml:lang="en">If the resource has been anonymized, this field can be used for entering more information, e.g., tool or method used for the anonymization, by whom it has been performed, whether there was any check of the results , etc.</xs:documentation><xs:appinfo><identifier>http://w3id.org/meta-share/meta-share/anonymizationDetails</identifier><label xml:lang="en">anonymization details</label></xs:appinfo></xs:annotation><xs:complexType><xs:simpleContent><xs:restriction base="ms:langString"><xs:maxLength value="1000"/></xs:restriction></xs:simpleContent></xs:complexType></xs:element>