四分律行事鈔資持記 3.17

3.17

四通相中。文舉縵衣為例。即如三衣互為從衣，但是約用加名，不論體相。然雖互通，本制須定；有闕開從，故云非無大分等。

Fourth, in [the section] "Common Characteristics": The text adduces the unpatched robe (māndhāra) as an example. That is, just as the three robes serve mutually as secondary robes, it is merely applying a name according to use, without regard to their substance or form. However, although they are mutually interchangeable, the original prohibition must remain fixed; when there is a deficiency, using another is permitted as an exception, hence it says "not without a general division" and so on.