How does a cost item retain a link to the types of IfcElementQuantities it is interested in?

Originally brought up https://forums.buildingsmart.org/t/how-does-a-cost-item-retain-a-link-to-the-types-of-ifcelementquantities-it-is-interested-in/3565

Given this situation:

1. A cost item is created.
2. A product is controlled by the cost item via `IfcRelAssignsToControl`.
3. The user selects which quantity set name (e.g. `Qto_WallBaseQuantities`) and quantity name (e.g. `NetVolume`) they want to relate to the `CostQuantities` list.
4. The model changes. More walls are added to the model, and they are assigned to the cost item via `IfcRelAssignsToControl`.

Now, is the user expected to again go through the process of selecting the applicable quantity set name and quantity name? Isn't this impractical?

Do we expect some walls to have their quantity taken from `GrossVolume` and some from `NetVolume`? Do we expect quantities from different objects to be added? E.g. some from `Qto_WallBaseQuantities` and some from `Qto_ColumnBaseQuantities`? Must the user write some filter saying "just quantify any quantity named "NetVolume" regardless of the quantity set name? The filter cannot be stored anywhere, so the viewing software will not know what determined what quantity take off if they are doing design authoring.