What is central to the book’s definition of sunnah here?

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

• An isolated act by an unknown person
• Consistent practice of the Prophet or the rightly guided caliphs
• Every act supported only by decisive fard proof
• Every act that must be avoided