gml:VerticalDatum is a textual description and/or a set of parameters identifying a particular reference level surface used as a zero-height surface, including its position with respect to the Earth for any of the height types recognized by this International Standard.
The attribute gml:id supports provision of a handle for the XML element representing a GML Object. Its use is mandatory for all GML objects. It is of XML type ID, so is constrained to be unique in the XML document within which it occurs.
Source
<element name="VerticalDatum" type="gml:VerticalDatumType" substitutionGroup="gml:AbstractDatum"><annotation><documentation>gml:VerticalDatum is a textual description and/or a set of parameters identifying a particular reference level surface used as a zero-height surface, including its position with respect to the Earth for any of the height types recognized by this International Standard.</documentation></annotation></element>