四分律行事鈔資持記 3.69

3.69

次科。先三衣說淨者，後衣法成，前衣自失，即入淨故。不捨吉者，違制罪也。

In the next heading: declaring the purification of the previous three robes is because when the ritual for the subsequent robe succeeds, the previous robe is automatically lost and immediately enters [the status of] purified property. That failing to relinquish it incurs a duṣkṛta is an offense of violating the prohibition.