俱舍論記 20.118

20.118

「論曰至故名微細」者，此釋眠義。根本十惑現在前時，行相難知，故名微細。猶如睡眠行相微細，故名為眠。

Regarding "The commentary states: [...] hence called subtle": this explains the meaning of "dormancy" (śaya/mian). When the ten fundamental afflictions are currently present, their modes of activity are difficult to know, and therefore they are called subtle. Just as when asleep (svapna), the modes of activity are subtle, so they are called "dormancy" (mian).