俱舍論記 5.158

5.158

「謂法作用至一法有用」者，釋功能，即是八法作用。或名士用，士謂士夫，如士夫用也，從喻為名。本相於八，隨相於一，各有用也。

Regarding "It refers to the activity of dharmas ... one dharma has efficacy": this explains function, which is the activity of the eight dharmas. Alternatively, it is called masculine effort (puruṣakāra); "masculine" refers to a man, like a man's exertion, named by way of metaphor. The primary characteristic has efficacy regarding eight, and the secondary characteristic has efficacy regarding one.