An Arc is an arc string with only one arc unit, i.e. three control points including the start and end point. As arc is an arc string consisting of a single arc, the attribute "numArc" is fixed to "1".
<element name="Arc" type="gml:ArcType" substitutionGroup="gml:ArcString"><annotation><documentation>An Arc is an arc string with only one arc unit, i.e. three control points including the start and end point. As arc is an arc string consisting of a single arc, the attribute "numArc" is fixed to "1".</documentation></annotation></element>