The space boundary defines the physical or virtual delimiter of a space by the relationship IfcRelSpaceBoundary to the surrounding elements. 

The IfcRelSpaceBoundary is defined as an objectified relationship that handles the element to space relationship by objectifying the relationship between an element and the space it bounds. It is given as a one-to-one relationship, but allows each element (including virtual elements and openings) to define many such relationship and each space to be defined by many such relationships. 

• 1st level space boundary: defined as boundaries of the space, not taking into account any change in building element or spaces on the other side.
• 2nd level space boundary: defined as boundary taking any change in building element or spaces on the other side into account. It can be further distinguished into:
  • 2nd level type A: There is a space on the other side.
  • 2nd level type B: There is a building element on the other side.

The exact definition of how space boundaries are broken down depends on the view definition, more detailed conventions on how space boundaries are decomposed can only be given at the domain or application type level.

• In an architectural or FM related view, a space boundary is defined totally from inside the space. This is a 1st level space boundary.
• In a thermal view, the decomposition of the space boundary depends on the material of the providing building element and the adjacent spaces behind. This is a 2nd level space boundary.

Space boundary at first level: 

Space boundary at second level: 

Space boundary at second level type A: 

Space boundary at second level type B: 

The differences between the 1st and 2nd level space boundaries is identified by:

• 1st level: SELFIfcRoot.Name = "1stLevel" SELFIfcRootDescription = NIL
• 2nd level: SELFIfcRoot.Name = "2ndLevel" SELFIfcRootDescription = "2a", or "2b"

As shown on the image below, if the IfcRelSpaceBoundary is used to express a virtual boundary, the attribute PhysicalOrVirtualBoundary has to be set to VIRTUAL. The attribute RelatedBuildingElement shall point to an instance of IfcVirtualElement. If the correct location is of interest, the attribute ConnectionGeometry is required. 

As shown on the image below, if the IfcRelSpaceBoundary is used to express a physical boundary between two spaces, the attribute PhysicalOrVirtualBoundary has to be set to PHYSICAL. The attribute RelatedBuildingElement has to be given and points to the element providing the space boundary. The attribute ConnectionGeometry may be inserted, in this case it describes the physical space boundary geometically, or it may be omitted, in that case it describes a physical space boundary logically. 

The IfcRelSpaceBoundary may have geometry attached. If geometry is not attached, the relationship between space and building element is handled only on a logical level. If geometry is attached, it is given within the local coordinate systems of the space. 

changes 

• IFC2X_FINAL changes:
  • The data type of the attribute_RelatedBuildingElement_ has been changed from IfcBuildingElement to its supertype IfcElement.
  • The data type of the attribute ConnectionGeometry has been changed from IfcConnectionSurfaceGeometry to its supertype IfcConnectionGeometry.
• IFC4 changes:
  • The attribute RelatedBuildingElement has been made mandatory. For virtual boundaries the reference to IfcVirtualElement is now mandatory.
  • The RelatedBuildingElement attribute is no longer optional.
  • Type of the RelatingSpace attribute changed from IfcSpace to IfcSpaceBoundarySelect.