The objectified relationship IfcRelDefinesByProperties defines the relationships between property set definitions and objects. Properties are aggregated in property sets. Property sets can be either directly assigned to occurrence objects using this relationship, or assigned to an object type and assigned via that type to occurrence objects. The assignment of an IfcPropertySet to an IfcTypeObject is not handled via this objectified relationship, but through the direct relationship HasPropertySets at IfcTypeObject. 

The IfcRelDefinesByProperties is an N-to-N relationship, as it allows for the assignment of one or more property sets to one or more objects. Those objects then share the same property definition.