The author begins a teaching list of 18 Wajib acts of prayer. A Wajib is legally binding, but it is not the same category as a Fard.
• Deliberately leaving a Wajib is sinful and requires the prayer to be repeated.
• Forgetfully leaving a Wajib normally requires Sujud al-Sahw.
• If required Sujud al-Sahw is also omitted, the prayer remains deficient and repetition is required under the stated Hanafi rule.
The commentary notes that the author's count of 18 is a teaching list; later jurists also discuss other Wajib details.