The last three levels complete a useful hierarchy:
• Wajib: deliberate omission is sinful; forgetful omission can require Sujud al-Sahw.
• Sunnah: completes the prayer and follows the transmitted method, but omission by itself does not normally require Sujud al-Sahw.
• Adab: refines focus, dignity and orderly performance.
Keeping these categories separate prevents every recommended posture from being treated as a validity condition.