I've been looking at pyvista which is based on vtk, and I'm a bit surprised that there is no order imposed in the internal data structure of the indices for the various cells of the geometric objects.
A tetrahedron is simply a tuple of 4 indices, a triangle is a tuple of 3. Often this is good, but for various operations like clipping/mesh boolean operations etc., I'd think it's natural to demand some sort of fixed order - i.e. always fixed clockwise order in the case of triangle/convex polygon. Based on how important this ordering is in discrete differential geometry, I'd think imposing order would be very useful for all sorts of things vtk can do, like geodesics etc.
So why isn't that the case? Is it not as useful as I think?
Simple: in 3D, there is no clockwise/couterclockwise order, you can look at a polygon from both sides.
(On a complete mesh, you can demand that all faces be oriented consistently, provided if forms an orientable manifold.)
Final remark: if such an order was possible, you can be sure that it would be in use everywhere.