IFC 4.3: Canonical way to provide mass/weight for IfcDistributionBoard (scope of Qto_*BaseQuantities vs. Psets; deprecate or widen?)

**Labels:** `question`, `clarification`, `Pset`, `MEP`

## Problem summary
In our electrical domain use cases (low-voltage switchboards, IFC4.3), we need to carry mass/weight for both the main distribution board and field sections. We attempted to use a BaseQuantities Qto intended for distribution boards (e.g., Qto_DistributionBoardBaseQuantities) but found it not applicable / not listed for IfcDistributionBoard in IFC 4.3. The bSI Validation Service flags our file when we attach that Qto to IfcDistributionBoard, even though the use case requires a board-level weight.
Our IDS checks attach *weight* (and other base quantities) to **board instances**. The bSI Validation Service flags this because the quantity set appears applicable only at the **type** level (or, depending on the table, not applicable to DistributionBoard at all). This blocks validation although the information is operationally needed at the occurrence.

## Why it matters
- Implementers need a stable, schema‑conform place for weight at the **installed board** for engineering, logistics and safety calculations.
- Current tables/docs give contradictory signals for where this Qto belongs, causing divergent validator behavior.

## Questions
1. In IFC4.3, what is the **authoritative applicability** of `Qto_DistributionBoardBaseQuantities`?
2. If it currently excludes `IfcDistributionBoard` **occurrences**, can we:
- (a) extend applicability to occurrences, and/or
- (b) align with a more generic quantity location (e.g., `Qto_ElementBaseQuantities.NetWeight` if applicable)?
3. Should any obsolete mapping be **deprecated** to prevent false positives?

## Minimal repro (conceptual)
- IFC file with `IfcDistributionBoard` (occurrence) + `IfcElementQuantity` named `Qto_DistributionBoardBaseQuantities` carrying `NetWeight`.
- Validation Service raises a rule: quantity set not allowed here.

## Expected vs. actual
- **Expected:** Weight accepted on the occurrence.
- **Actual:** Validation fails due to applicability constraints.

## Proposed resolution (for discussion)
- Either (A) widen applicability of the appropriate Qto_*BaseQuantities to IfcDistributionBoard, or (B) add/confirm a standard Pset for board mass/weight and document this clearly.
- Update Validation Service rules and example IDS to reflect the decision.
- If a legacy Qto is no longer intended for 4.3, mark as deprecated with migration note.

## Related spec
- IFC4.3 quantity sets & applicability tables.

## Proposed next steps
- Confirm the applicability table; if misaligned, update the PSet/Qto registry and add a normative note & example.
- Provide a migration/deprecation note for tools and validation rules.