4.1.3 Object Attributes

All entities having semantic significance derive from IfcRoot, where instances are identifiable within a data set using a compressed globally unique identifier (IFC-GUID). This identifier must never change during the lifetime of an object, which allows data to be merged, versioned, or referenced from other locations.

Resource-level instances (not deriving from IfcRoot) do not have any identity, such that two instances having identical state are considered equal. For example, if an object has coordinates described by an IfcCartesianPoint instance, another object with the same coordinates may have a separate instance of IfcCartesianPoint or share the same instance; such difference is a matter of data storage optimization and does not imply any semantic relationship. This also implies that non-rooted instances may only exist if referenced by at least one rooted instance through either a direct attribute or inverse attribute, or following a chain of attribute references on instances.

The distinction between rooted and non-rooted (resource-level) entities achieves several goals: * File size may be reduced by interning (sharing) non-rooted data instances; * Database retrieval may be more efficient by storing non-rooted data local to rooted data instances; * Storage size may be reduced by avoiding IFC-GUID storage for items not requiring direct retrieval; * Comparisons of differences may be done at a higher level where the context of such change is apparent; * Implementations may treat non-rooted data instances as immutable for efficiency or simplified usage.