The required domain attribute is the domain of the parameterization of the curve.
Source
<xs:element name="Polyline13Core" type="Polyline13CoreType" substitutionGroup="Curve13Core"><xs:annotation><xs:documentation>The global Polyline13Core element is the mathematical core of a 3D polyline. The polyline is a series of connected linear segments.</xs:documentation></xs:annotation></xs:element>