Editing Industry Foundation Classes (section)

===IfcPropertyDefinition===
IfcPropertyDefinition captures dynamically extensible property sets.  A property set contains one or more properties which may be a single value (e.g. string, number, unit measurement), a bounded value (having minimum and maximum), an enumeration, a list of values, a table of values, or a data structure.  While IFC defines several hundred property sets for specific types, custom property sets may be defined by application vendors or end users.
* IfcPropertySet represents a set of properties attached to an object occurrence or object type. 
* IfcPropertySetTemplate [IFC2x4] captures definitions of properties and their data types.