瑜伽師地論略纂 12.140

12.140

論云「與清淨住為前道故，當言此住名不染污；猶未得故，當言此住隨雜染污」者，第八地中根本無相，此第七地初得無相，是第八地前導，故名不染；未得根本無相，尚有功用，有所希求，未能任運無相，故隨雜染。

The treatise says: "Because it serves as the preceding path for the pure abode, one should say this abode is called undefiled; because it has not yet been attained, one should say this abode is defiled along with the mixed." In the eighth bhūmi there is the fundamental signless; this seventh bhūmi first attains the signless and is the precursor to the eighth bhūmi, hence it is called undefiled. Not having attained the fundamental signless, it still involves effort and has something to seek, not yet being able to be spontaneously signless, and hence it is defiled along with the mixed.