Dear OCCT Team,
I find the Note in the Modeling Data document Page 31:
that this data structure does not contain any back references. All references go from more complex underlying shapes to less complex ones.
For example, If I have a vertex and want to know which Edge contains it, I can use
TopExp::MapShapesAndAncestors() to find it.
If there is back reference in the TopoDS_Shape, then I access the parent shape (here is the edge) more convenient and straightforward.