俱舍論疏 25.24

25.24

論「於中後四至最居下故」，明五中退法一種無退姓義，以最下故。餘四皆容退性義，以即下姓可容退故。

On the Śāstra's statement: "Among them, the latter four ... because it occupies the lowest position": This explains that among the five, the one liable to fall away does not admit of falling away from its lineage, because it is the lowest. The other four all admit of falling away from their lineages, because there is an inferior lineage to which they can fall away.