IFC structure - IfcProject, IfcSite, IfcFacility

The IFC schema has a clear visible structure by default, which is displayed similarly in all IFC viewers:

This structure is also documented in the current version of the buildingSMART documentation: Link official IFC4.3 Documentation - IfcFacility.
As visible in the documentation, the relationships and dependencies in the background of the schema are far more complex and relevant for a better understanding of the IFC data schema:

When exporting an IFC from Revit, the first 3 levels of the visible IFC structure are populated with information from the Project Information:

IfcProject

The top level container carries general information about the project (e.g. name of the project, units, the world coordinate system, information about the map conversion, orientation of the geographic north).
This information is populated based on the Revit project information:

Additional information can be mapped via the shared parameters (see separate blog post).

IFC 4.3 Documentation - IfcProject

IfcSite

IfcSite represents the property or the area of land and carries general information, such as the local coordinate system (see separate blog post).
This information is populated from Revit project information as follows and can be expanded using additional parameters:

IFC4.3 Documentation - IfcSite

IfcBuilding

IfcBuliding represents the actual building itself and is also a spatial container populated with information from the Revit Project Information:

IFC4.3 Documentation - IfcBuliding

As already mentioned, since IFC4.3, other types of IfcFacility can be used when working on Infrastructure projects. This option can also be used when exporting IFC 4.3 from Revit:

An IfcProject can have several instances of IfcSite or IfcBuilding . As these spatial containers are populated based on the project information, Revit only supports one instance of each of these classes during export - this means that it is currently not possible to create several instances of IfcSite or IfcBuilding in one Revit project. IfcSite or IfcBuilding to define.

An exception to this is the export of linked Revit projects - these can either be exported to the same IfcProject (resulting in 2 IfcSites with 2 IfcBuildings) or to the same IfcSite (resulting in 2 IfcBulidings on the same IfcSite). You can find out more in the Autodesk IFC Manual.

Find out more in this video: