A person starts a voluntary prayer, but the prayer is invalidated before completion. The phrase “ṣalāt al-taṭawwuʿ” expressly brings such prayer within the make-up rule.
“Qaḍāhumā” refers to both named forms of worship, not only fasting. Detailed Hanafi questions about rakʿahs and different types of interrupted prayer belong to the prayer chapters, beyond this sentence.