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The purpose of PnIndex in the IfcTriangulatedFaceSet

Hi Experts,

I would like to know the purpose of PnIndex in the IfcTriangulatedFaceSet.

As shown in the image below, I could use CoordIndex directly to find the points in the CoordList, because the list of PnIndex is a list with ascending number (1, 2, 3, 4……). Is PnIndex not necessary to my case?
image
For my case, one IfcTriangulatedFaceSet has its corresponding IfcCartesianPointList3D. Could several
IfcTriangulatedFaceSets share the same IfcCartesianPointList3D by using PnIndex?