IfcLinearPlacement.PlacementMeasuredAlong position have to be clarified

IfcLinearPlacement.PlacementMeasuredAlong references to curve, but what coordinate system is used for the curve?
I have to search IfcProduct that has the curve in Representations and use ObjectPlacement to have answer.
If this is curve referenced by IfcAlignment.Axis - I possible have to use its ObjectPlacemenyt but see no specification for this at all.

Some possible solutions:
- PlacementMeasuredAlong have to point to IfcAlignment
- IfcLinearPlacement.PlacementRelTo gives the curve placement - it must be clarified in IFC spec.... but it gives ability for inconsistent data.