Update IfcReferent.md

[RickBrice]

IfcRelNests.RelatedObjects is an ordered list. This change provides clarity that IfcReferent is to precede IfcAlignmentHorizontal in the ordered list. Such clarity does not currently exist in the documentation.

[SergejMuhic]

These are two different usages on IfcAlignment, so they should use two instances of IfcRelNests. Also, IfcReferent is not the entity responsible for this usage, so if needed, it should be on the Referent Nesting usage on IfcAlignment, but in all honesty, IMHO this is superfluous.

[RickBrice]

I agree that referents and alignment layouts are two different usages. Two instances of IfcRelNests should be used. However, there is not a definitive statement to this effect as far as I can see. Since not forbidden, referents and layouts are permitted to be in the same ordered list and the ordering rules must be cleared stated.

I would prefer a new rule that clearly states each IfcRelNests can only nest entities of the same usage. I didn’t approach it this way because a new rule would not be an editorial change and we would also need to define what it means for entities to belong to the same usage.

Your position is sound. However it relies on implied knowledge of IFC rather than explicit language in the specification.

[SergejMuhic]

I do not want to go down the rabbit hole but if you say "Your position is sound. " then I would again reference good principles in putting together technical documentation. Not everything has to be explicitly stated if the information can be derived from other resources.

My position does not rely on implied knowledge, this is the reason for a General Usage mvdxml.

But, if this needs to be explicitly stated, I do not see any problem in adding it. I would argue the correct place though. As stated above, "if needed, it should be on the Referent Nesting usage on IfcAlignment". Or even better, on the Object Nesting concept template. Maybe as far up as section "4 Fundamental Concepts and Assumptions". That way you stick to editorial changes.

[RickBrice]

Placing the suggested information in 5.4.3.1.5, Object Nesting concept for IfcAlignment is agreeable.