俱舍論記 7.127

7.127

「色界染心至故名得六」者，明色界染心得六，由二位得：一界退還、二起惑退。欲無覆通果心及色界善無覆心，由無色界退還得。色染二緣得，一由界退得、二由起惑退得。無色有覆及學心，但由惑退得，故名得六。

Regarding the passage from "A defiled mind of the form realm" up to "therefore it is called acquiring six": this clarifies that a defiled mind of the form realm acquires six, which is acquired through two positions: first, retrogressing and returning from a realm; second, retrogressing by giving rise to defilements. The desire-realm unobstructed fruition-of-mastery mind and the form-realm wholesome and unobstructed minds are acquired by retrogressing and returning from the formless realm. Defilement of the form [realm] is acquired through two conditions: first, acquired by realm-retrogression; second, acquired by retrogressing through giving rise to defilements. The obstructed [mind] of the formless [realm] and the trainee mind are acquired only through defilement-retrogression; therefore it is called acquiring six.