What is the logic of requiring the axis of an IfcDistributionPort to point in the flow direction?

The docs for IfcDistributionPort state:

> The Axis points in the direction of the physical connection away from the product if FlowDirection equals SOURCE (or SOURCEANDSINK or NOTDEFINED), or points opposite direction (to the product) if the FlowDirection equals SINK.

What is the logic for this? This is the logic currently provided:

> NOTE The rationale for positioning the Axis in the direction of flow is to allow for the same geometry to be used, such as for connectors with polarized cross-section.

What geometry is this referring to? Is it suggesting that an IfcDistributionPort can have geometry?

However, that contradicts this sentence on the same page:

> Ports are similar to openings in that they **do not have any visible geometry**; such geometry is captured at the shape representation of the enclosing element or element type.

The sentence is also weird to me because openings do have geometry, I guess invisible geometry. So maybe a port is also invisible geometry? What would be an example of what a port would look like? Every port I've seen implemented has no geometry.