The required domain attribute is the domain of the parameterization of the curve.
Source
<xs:element name="Aggregate12Core" type="Aggregate12CoreType" substitutionGroup="Curve12Core"><xs:annotation><xs:documentation>The global Aggregate12Core element is the mathematical core of a 2D aggregate curve. The aggregate curve is a sequence of parametric sub-curves.</xs:documentation></xs:annotation></xs:element>