The valid format of a reference to a VO-DML element. (Used to be 'UTYPE'). MUST have a prefix that elsewhere in the VOTable is defined to correspond to a VO-DML model defining the referenced element. See "mapping document", https://volute.g-vo.org/svn/trunk/projects/dm/vo-dml/doc/UTYPEs-WD-v1.0.pdf. Suffix, separated from the prefix by a ':', MUST correspond to the vodml-id of the referenced element in the VO-DML/XML representation of that model. Identifies an alternative type for the element containing the annotation. MAY be specified if the vodml-type attribute is set. In that case MUST reference a super type of the VO-DML type identified by that attribute. There may be more rules associated to this, see the "mapping document". MUST be provided on annotations of FIELDref and PARAMref and on GROUP and PARAM if these are contained in a VO-DML annotated GROUP. If not provided on a root GROUP, indicates that it represents a stand-alone object and then the vodml-type MUST be available. MUST reference the exact VO-DML type of the instance represented by the container of the VODMLAnnotation element. Q: should use="required"? MUST reference the exact VO-DML type of the instance represented by the container of the annotation element. Q: should we make use=required? MUST be provided on annotations of FIELDref and PARAMref and, if contained in a VO-DML annotated GROUP or PARAM. If not provided on a root GROUP, indicates that it represents a stand-alone object. Identifies an alternative type for the instance represented by the instance or value containing the annotation. MUST reference a super type of the VO-DML type identified by the vodml-type attribute if that exists. The more types up the inheritance hierarchy are listed here, the easier it is to find instances of said type using simple statements such as XPath. MUST reference the exact VO-DML type of the instance represented by the container of the annotation element. Q: should we make use=required? MUST be provided on annotations of FIELDref and PARAMref and, if contained in a VO-DML annotated GROUP or PARAM. If not provided on a root GROUP, indicates that it represents a stand-alone object. MUST be provided if the FIELDref is part of a GROUP that has a VO-DML element. The VO-DML annotation MUST in that case identify a VO-DML Role (be it as attribute or as element). For other possible rules see the "mapping document". MUST be provided if the PARAM is part of a GROUP that has a VO-DML element. The VO-DML annotation MUST in that case identify a VO-DML Role (be it as attribute or as element). For other possible rules see the "mapping document". MUST be provided if the PARAMref is part of a GROUP that has a VO-DML element. The VO-DML annotation MUST in that case identify a VO-DML Role (be it as attribute or as element). For other possible rules see the "mapping document".