俱舍論記 24.37

24.37

「二上流中至行色界者」者，明二上流差別不同。二上流中，前觀、後止，樂慧、樂定有差別故。二上流者二生已上，於下地中得般涅槃，見不違理，而言此往色究竟天及有頂天為極處者，由此過彼無行處故。如預流者極七返生，於中亦有一、二生等。又總結言：此五名為行色界者。樂定上流雖生有頂，曾經色界生故，亦名行色界，是五不還中上流攝也。

Regarding “Among the two upward-streamers [...] those who fare in the form realm”: this clarifies the differences between the two upward-streamers. Among the two upward-streamers, the former emphasizes vipaśyanā and the latter śamatha; there is a distinction between delighting in wisdom and delighting in samādhi. For the two upward-streamers, attaining parinirvāṇa in a lower stage after two or more births is seen not to contradict principle, but stating that their ultimate destinations are Akaniṣṭha Heaven and the Peak of Existence is because beyond those there is no place to proceed. Just as a stream-enterer has at most seven returns, yet among them there are also those with one or two births, etc. Again, it summarizes: these five are called those who fare in the form realm. Although the upward-streamer delighting in samādhi is born in the Peak of Existence, because they have previously experienced birth in the form realm, they are also called faring in the form realm, and are subsumed under the upward-streamer among the five non-returners.