The optional label attribute is the model entity "nameplate". Normally it can be seen at the entity item in the project tree.
Source
<xs:complexType name="Curve12BaseType" abstract="true"><xs:annotation><xs:documentation>The Curve12BaseType is the abstract base type for all geometric 2D curves (curve(t):R1->R2) present in the CAD scene.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="GeometryBaseType"/></xs:complexContent></xs:complexType>