瑜伽論記 1.212

1.212

五識地中，所以不說恒行意者，以五識地各別有同時色根，意地無別同時根，故說末那名恒行意。是故六識皆有同時根及次第滅意根也。

The reason why constantly functioning manas is not mentioned in the stage of the five consciousnesses is that the stage of the five consciousnesses each distinctly has simultaneous material faculties; the mental stage has no distinct simultaneous faculty, so it explains manas as the constantly functioning manas. Therefore, the six consciousnesses all have simultaneous faculties and immediately preceding ceased mental faculties.