俱舍論記 6.131

6.131

「毘婆沙師至名異熟因」者，答。毘婆沙師，因是善惡、果是無記，異類而熟是異熟義。謂異熟因唯異類熟。俱有、相應、同類、遍行因唯同類熟。能作一因，兼同、異熟。故唯此一名異熟因。

Regarding "The Vaibhāṣikas [say]... is called the cause of maturation": this is the answer. For the Vaibhāṣikas, the cause is wholesome or unwholesome, while the fruit is indeterminate; ripening as a different category (vi-pac) is the meaning of vipāka. That is, the cause of maturation ripens only as a different category. Coexistent, associated, similar, and universal causes ripen only as the same category. The efficient cause (kāraṇa-hetu) alone combines both same-category and different-category ripening. Therefore, only this one is called the cause of maturation.