IfcRelConnectsPorts RealizingElement usecases are not clear in the documentation

In the docs for IfcDistributionPort it describes how an IfcPipeSegment can be used as the RealizingElement for the port connection.

In the docs for IFC2X3 docs for IfcDistributionPort it talks about IfcFlowFittings like flanges or couplings can be a realizing element:

> This connection may be physically realized with a specific fitting that is used to physically connect the duct segment and the tee fitting, such as a flange or coupling.

In IFC4, the data type is IfcElement. Can I have an IfcWall as a realising element? An IfcChiller? Or only segments and fittings?

What is the rule when to use a realizing element and when not to?

Start by comparing the IfcDistributionPort documentation with the IFC2X3 IfcDistributionPort documentation, focusing on the RealizingElement descriptions quoted in the issue. Determine and document the rule for when RealizingElement is used and which element types, such as fittings, segments, walls, or chillers, are valid.

Written by the indexing model from the issue text.