How does a cost item know that it is counting a controlled product?

https://forums.buildingsmart.org/t/how-does-a-cost-item-know-that-it-is-counting-a-controlled-product/3564

A cost item may be controlling zero or more products. If the purpose of the cost item is to quantify the units of that product (e.g. count how many products there are), then the `CostQuantities` attribute should include an `IfcPhysicalQuantity` that reflects the count.

There are two options:

1. Put a `IfcQuantityCount` in the `CostQuantities` manually, and fill in the `CountValue` manually. The problem is that this is manual. The software does not know that this actually reflects counting a set of items.
2. Assign a `IfcPhysicalQuantity` that is part of a `IfcElementQuantity` of all of the products controlled by the `IfcCostItem`. The problem is that no standard quantity set (i.e. `Qto_*`) contains any count values. It would always be set to 1.

Thoughts? How does a software know that a cost item is counting the product it controls?