八識規矩補註 1.38

1.38

此言第六識有情動身發語時，於八識中行相最勝，故《成業論》云：「由外發身語，表內心所思，譬如潛淵魚鼓，波而自表。」此識俱思而有三種，謂審慮思、決定思、動發勝思，餘識所無，故最勝也。

This means that when sentient beings move their bodies and issue speech, the sixth consciousness has the most prominent cognitive form among the eight consciousnesses. Therefore the Karmasiddhiprakaraṇa states: 'Originating outwardly as body and speech, it expresses what is thought inwardly by the mind, just as a fish diving in a pool stirs waves to express itself.' The intention (cetanā) associated with this consciousness has three kinds: deliberative intention, decisive intention, and preeminent intention that sets into motion; since the other consciousnesses lack these, it is the most preeminent.