An IfcObjectDefinition is the generalization of any semantically treated thing or process, either being a type or an occurrences. Object definitions can be named, using the inherited Name attribute, which should be a user recognizable label for the object occurrence. Further explanations to the object can be given using the inherited Description attribute. A context is a specific kind of object definition as it provides the project or library context in which object types and object occurrences are defined. 

Objects are independent pieces of information that might contain or reference other pieces of information. There are four essential kinds of relationships in which object definitions (by their instantiable subtypes) can be involved: 

• Assignment of other objects - an assignment relationship (IfcRelAssigns) that refers to other types of objects and creates a bi-directional association. The semantic of the assignment is established at the level of the subtypes of the general IfcRelAssigns relationship. There is no dependency implied a priori by the assignment.
• Association to external resources - an association relationship (IfcRelAssociates) that refers to external sources of information (most notably a classification or document) and creates a uni-directional association. There is no dependency implied by the association.
• Aggregation of other objects - an aggregation relationship (IfcRelAggregates) that establishes an unordered, spatial whole/part relation and creates a bi-directional relation. There is an implied dependency established.
• Nesting of other objects - a nesting relationship (IfcRelNests) that establishes an ordered, non-spatial whole/part relation and creates a bi-directional relation. There is an implied dependency established.
• Declaration within a context - a relationship (IfcRelDeclares) of the uppermost object definition within the object definition tree (e.g. the summary object within an object nesting tree) to the context (a project or project library). It applies the units, representation context and other context information to this object definition and all dependent ones.

changes 

• IFC4 changes:
  • The new subtype IfcContext and the relationship to context HasContext has been added.
  • The decomposition relationship is split into ordered nesting (Nests, IsNestedBy) and un-ordered aggregating (Decomposes, IsDecomposedBy).
  • Name of the inverse attribute Decomposes changed to IsNestedBy.
  • Type of the inverse attribute Decomposes changed from "SET OF [0:1] IfcRelDecomposes FOR RelatedObjects" to "SET OF [0:?] IfcRelNests FOR RelatingObject".
  • Name of the inverse attribute IsDecomposedBy changed to Nests.
  • Type of the inverse attribute IsDecomposedBy changed from "SET OF [0:?] IfcRelDecomposes FOR RelatingObject" to "SET OF [0:1] IfcRelNests FOR RelatedObjects".