How do you designate spare parts for facility management?

Originally here: https://forums.buildingsmart.org/t/how-do-you-designate-spare-parts-for-facility-management/3585

Potentially now a non-issue, but recording in case it still make sense.

I would assume some form of `IfcResource` subclass? Through elimination, spare parts may belong to different types of resources, such as IfcConstructionMaterialResource or IfcConstructionProductResource. However, there is no clear attribute which helps distinguish this as being a spare part instead of a resource that is not a spare part.

Perhaps it may be assumed through the absence of a `Usage` attribute (i.e. set to NULL), but it would be better for something explicit.

Another potential solution is to include all spare parts into its own IfcInventory, but it lacks a dedicated PredefinedType for it.