成唯識論演祕 7.163

7.163

五事中「相」。然三解脫有能、所知；相多所知，少通能知。所知、能知俱三解脫。

This refers to "sign" among the five factors. However, the three liberations have that which knows and that which is known; "sign" mostly pertains to the known, and marginally to that which knows. Both the known and that which knows are the three liberations.