俱舍論疏 25.21

25.21

論「若執退者至故唯有二」，明若退等定退者，即唯欲界具六種性，不應通上界，以是無退等故。

On the Śāstra's statement: "If one maintains that the liable to fall away ... therefore there are only two": This explains that if those liable to fall away definitely fell away, then only the desire realm would possess all six lineages and they could not extend to the higher realms, because [in the higher realms] there is no falling away and so forth.