A fard obligation rests on decisive evidence, with special consequences for performance and denial. A wajib obligation is binding in practice but may rest on evidence that is not definitive.
Sunnah has recognized normative force as Prophetic practice, whereas nafl is an additional devotional act without the status of fard or wajib. Emphasized and non-emphasized Sunnah should also be distinguished.
Hanafi discussions of al-Fatihah in prayer and the witr prayer illustrate why binding practical force and the certainty of proof are different criteria.
An act should not be classified as fard or wajib merely because ordinary speech calls it “important” or “necessary.”