The cubic transition segment is modelled with IfcPolynomialCurve with certain restrictions. The size of the attribute CoefficientsY is restricted to 4. The cubic term is on position 4. In general, the CoefficietsX size would be restricted to 2 and values specified as [0, 1]. This produces the following result:
y = CoefficientsX*x³, where other values in CoefficientsY are 0.
The following diagram shows the generic classes and relationships used when applying this concept.