IfcPropertySetDefinition is a generalization of all individual property sets that can be assigned to an object or type object. The property set definition can be either:

• Dynamically extendable property sets - IfcPropertySet, a set of properties for which the IFC specification only provides a kind of "meta model", to be further declared by agreement. This means that no entity definition of the properties exists within the IFC specification. The declaration is done by assigning a significant string value to the Name attribute of the entity as defined in the entity IfcPropertySet and at each subtype of IfcProperty, referenced by the property set. Dynamically defined property sets can have an underlying template provided by IfcPropertySetTemplate.
• Statically defined property sets - IfcPreDefinedPropertySet, a property set entity that exists within the IFC specification. The semantic meaning of each statically defined property set is declared by its entity type and the meaning of the properties is defined by the name and data type of the explicit attribute representing it.

Property set definitions define information that is shared among multiple instances of objects, either object occurrences or object types. IfcPropertySetDefinition's (by their instantiable subtypes) can participate within the following relationships:

• Assignment to object types - a DefinesType direct relationship to IfcTypeObject that applies the property set, with all included properties, to the object type. Those properties apply to all object occurrences having the same object type.
• Assignment to object occurrences - an DefinesOccurrence relationship to IfcRelDefinesByProperties that applies the property set, with all included properties, to the object occurrence.

changes 

• IFC4 changes:
  • The subtype IfcPreDefinedPropertySet has been added.
  • Name of the inverse attribute DefinesType changed to IsDefinedBy.
  • Type of the inverse attribute DefinesType type changed from "SET OF [0:1] IfcTypeObject FOR HasPropertySets" to "SET OF [0:?] IfcRelDefinesByTemplate FOR RelatedPropertySets".
  • Name of the inverse attribute PropertyDefinitionOf changed to DefinesType.
  • Type of the inverse attribute PropertyDefinitionOf changed from "SET OF [0:1] IfcRelDefinesByProperties FOR RelatingPropertyDefinition" to "SET OF [0:?] IfcTypeObject FOR HasPropertySets".