Editing Industry Foundation Classes (section)

===IfcRelationship===
IfcRelationship captures relationships among objects.  There are five fundamental relationship types: composition, assignment, connectivity, association, and definition.
* IfcRelDecomposes captures a whole-part relationship having exclusive containment such as subdividing a building into floors and rooms or a wall into studs and sheathing. 
* IfcRelAssigns captures assignment relationships where one object consumes the services of another object, such as a labor resource assigned to a task, or a task assigned to a building element.  
* IfcRelConnects indicates connectivity between objects such as a floor slab connected to a beam or a pipe connected to a sink.  
* IfcRelAssociates indicates external references for an object such as an external IFC library file where an object is defined.  
* IfcRelDefines indicates an instance-of relationship such as a pipe segment being of a particular type.