Accepted interpretation of IfcQuantityWeight.WeightValue

## Question
[8.17.3.10 IfcQuantityWeight - IFC 4.3.2 Documentation](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcQuantityWeight.htm)

What is the accepted interpretation of IfcQuantityWeight.WeightValue? Weight is a force but the data type for WeightValue is IfcMassMeasure. WR21 states “There is no distinction between the concept of Mass and Weight in the current IFC Release”.

We know from basic science that Force = Mass * Acceleration, F = m * a

The gravitational acceleration of Earth is a = g = 32.2 ft/s/s = 9.81 m/s/s.

I am trying to define the Qto_BeamBaseQuantities.GrossWeight for an IfcBeam. The gross weight is 0.1 kip =100 lbf (Imperial Units) = 444.8 N (SI Units).

The basic relationship between weight and mass are:
100 lbf = 3.106 lbm * 32.2 ft/s/s = 100 slug * 1 ft/s/s (lbf = pound of force, lbm = pound of mass)
444.8 N = 45.34 kg * 9.81 m/s/s = 444.8 kg * 1 m/s/s

Am I supposed to assume a unit acceleration (1 ft/s/s or 1 m/s/s) so the magnitude of the mass value is equal to the magnitude of the weight value? Or assume Earth’s gravitational acceleration and use the corresponding mass?

If my model has IfcProject.UnitsInContext defined for Imperial units with length=FOOT, mass=POUND, time=SECOND, and unit acceleration is assumed, IfcQuantityWeight.Unit will be needed for conversion between pound-mass (lbm) and slug. If Earth's gravitational acceleration is assumed, IfcQuantityWeight.Unit is not needed. Misinterpretation of this can lead to weight values (as a force) being wrong by a factor proportional to 32.2.

Mass and force are not equivalent. The correct scientific distinction should be made and implemented in IFC 4.4. The data type for WeightValue should be changed to IfcForceMeasure.

Start with the linked IfcQuantityWeight documentation and the WR21 statement, then compare Qto_BeamBaseQuantities.GrossWeight with IfcProject.UnitsInContext. Review the six-comment discussion for the current interpretation and its implications for IFC 4.4. Done means documenting an accepted interpretation or reaching agreement on whether WeightValue should use IfcForceMeasure.

Written by the indexing model from the issue text.