The entity definition for IfcAlignmentVerticalSegment (220.127.116.11.1) says
parabolic arc segment - IfcAlignmentVerticalSegmentTypeEnum.PARABOLICARC. which can describe symmetric and unsymmetric parabolas.
How is an unsymmetric parabola defined? It seems the HorizontalLength attribute is insufficient since the horizontal length on either side of the PVI (L1 and L2 in the image below) is needed to define the asymmetry.