俱舍論記 24.38

24.38

「行無色者至成六不還」者，上來釋前六句。此釋第七句，明無色界四種不還。行無色者差別有四，謂在欲界從此命終，不生色界超生無色。此中差別唯有四種，由生般等有差別故。於前五內唯除中般，此四合名無色不還。此無色一，并前五種，成六不還。

Regarding “Those who fare in the formless [...] making six non-returners”: the preceding explained the first six lines. This explains the seventh line, clarifying the four kinds of non-returners in the formless realm. Those who fare in the formless realm have four varieties: dying here in the desire realm, they do not take birth in the form realm, but leap over to be born in the formless realm. The distinctions here are only of four kinds, due to the distinctions among attaining parinirvāṇa upon rebirth (upapatti-parinirvāyin) and so forth. Among the previous five, only the intermediate-attainer (antarā-parinirvāyin) is excluded; these four together are called the formless non-returners. This one formless category, together with the previous five kinds, makes six non-returners.