An ArcByBulge is an arc string with only one arc unit, i.e. two control points, one bulge and one normal vector.As arc is an arc string consisting of a single arc, the attribute "numArc" is fixed to "1".
<element name="ArcByBulge" type="gml:ArcByBulgeType" substitutionGroup="gml:ArcStringByBulge"><annotation><documentation>An ArcByBulge is an arc string with only one arc unit, i.e. two control points, one bulge and one normal vector. As arc is an arc string consisting of a single arc, the attribute "numArc" is fixed to "1".</documentation></annotation></element>