An IfcObject is the generalization of any semantically treated thing or process. Objects are things as they appear - i.e. occurrences. 

Objects are independent pieces of information that might contain or reference other pieces of information. There are several relationships in which objects can be involved:

• Association to external/internal resource information - an association relationship that refers to external/internal sources of information. See supertype IfcObjectDefinition for more information.
• Assignment of other objects - an assignment relationship that refers to other types of objects. See supertype IfcObjectDefinition for more information.
• Aggregation of other objects - an aggregation relationship that establishes a whole/part relation. Objects can either be a whole, or a part, or both. See supertype IfcObjectDefinition for more information.
• Assignment of a type : IsTypedBy - a definition relationship IfcRelDefinesByType that uses a type definition to define the common characteristics of this occurrences, potentially including the common shape representation and common properties of all object occurrences assigned to this type. It is a specific - occurrence relationship with implied dependencies (as the occurrence properties depend on the properties of the type, but may override them).
• Assignment of a partial type : IsDeclaredBy, Declares - a definition relationship IfcRelDefinesByObject that uses a component of a type definition (a part of a type, called the "declaring part") to define a component of an occurrence (part of occurrence, called the "reflected part"). This is also referred to as a "deep copy". The common characteristics of all parts in the occurrence are defined by parts in the type. It is a specific - occurrence relationship with implied dependencies (as the occurrence properties depend on the properties of the type, but may override them).
• Assignment of property sets : IsDefinedBy - a definition relationship IfcRelDefinesByProperties that assigns property set definitions to the object occurrence.

changes 

• IFC4 changes:
  • Name of the inverse attribute IsDefinedBy changed to IsDeclaredBy.
  • Type of the inverse attribute IsDefinedBy changed from "SET OF [0:?] IfcRelDefines FOR RelatedObjects" to "SET OF [0:1] IfcRelDefinesByObject FOR RelatedObjects"