Under 184.108.40.206 IfcStructuralCurveMember we find this paragraph:
The material of direct instances IfcStructuralCurveMember (in contrast to instances of the subtype IfcStructuralCurveMemberVarying) is defined by IfcMaterialProfileSetUsage and attached by the IfcRelAssociatesMaterial.RelatingMaterial . It is accessible by the inverse HasAssociations relationship. Composite profile beams can be represented by refering to several IfcMaterialProfiles within the IfcMaterialProfileSet that is referenced from the IfcMaterialProfileSetUsage. In case of tapered members, the material profile usage subtype IfcMaterialProfileSetUsageDual is used which specifies IfcMaterialProfileSets separately at the start and the end of the tapered member.
The material (IfcMaterial) in each IfcMaterialProfile( Set ) is specified minimally by a name which corresponds with an agreed upon standardized structural material designation. An external reference to the source which specifies the material designation should be provided. Alternatively, structural material properties may be provided by means of IfcMechanicalMaterialProperties and IfcExtendedMaterialProperties .
IfcExtendedMaterialProperties do not exist.
Maybe this section of the documentation needs to be rewritten? It does not make sense to me.