What is the decisive feature in the definition of fard?

Prescribed Acts and Prohibitions: Fard, Wajib, Sunnah, Nafl, Haram, Makruh, Azimah and Rukhsah

• Establishment by authoritative but non-conclusive evidence
• Being a continuously observed practice of the Prophet or caliphs
• Being additional to the binding duties
• Establishment by conclusive proof free of relevant uncertainty