Comment by YuechenLi

Ok, I think I oversimplified it too much previously, the more accurate way to word the difference is that in OCCT/ACIS/Parasolid, NURBS and analytical surfaces are separate entities altogether, whereas SMLib, due to its heritage, uses NURBS as the underlying representation of everything and only use analytical surfaces as shortcuts, and even their analytical surfaces are built on top of NURBS.