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="GeometryBaseType" abstract="true"><xs:annotation><xs:documentation>The GeometryBaseType is the abstract base type for all geometric elements.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="NodeWithIdBaseType"/></xs:complexContent></xs:complexType>