瑜伽論記 17.214

17.214

又解：七者、離染故名清淨，異生故名不清淨，約生對涅槃等為句故。第九者、體離染故名清淨，此處不得見諦故名不清淨處生對有暇處為句故。

Another explanation says: The seventh is called pure because of having abandoned defilement, and called impure because they are ordinary worldlings, formulated as a proposition regarding birth versus nirvāṇa, etc. The ninth is called pure because its substance has abandoned defilement, and called birth in an impure place because one cannot see the truths in this place, formulated as a proposition regarding birth versus a realm of leisure.