成唯識論述記 3.361

3.361

述曰：初解變異，後釋分別。謂根、境、識三和合位，除未合時，故言和位。此三之上，皆有順生一切心所功能作用，名為變異。謂此三法居種子時，及未合前，皆無順生心所作用，於三合位功能乃生。既與前珠，說名變異，變異即是三體上用。正解變異體即三法，次解分別。

The Commentary states: First it explains modification, and later it clarifies discrimination. That is, at the stage where faculty, object, and consciousness are harmoniously united—excluding the time before they unite, hence it says 'the harmonized stage'—upon these three there are all functional capacities that conform to generating all mental factors, which is called 'modification' (pariṇāma). That is, when these three phenomena reside as seeds, and prior to their union, they have no function of conforming to generating mental factors; only at the stage of the union of the three do their capacities arise. Since it is different from before, it is called 'modification'; modification is precisely the function upon the substances of the three. Correctly explaining modification: its substance is the three phenomena themselves; next, it explains discrimination.