俱舍論記 28.32

28.32

「雖一境性至非非想處」者，釋離下地約生不同分四。雖一境性四無色定體相無差別，離下地染生彼上地，約生不同故分四種。謂若已離第四靜慮染生上地時，立空無邊處。乃至已離無所有處染生上地時，立非想非非想處。

As for "Although it is one-pointedness of mind... the stage of neither-perception-nor-non-perception": this explains the division into four according to rebirth through detachment from lower stages. Although one-pointedness of mind has no difference in intrinsic characteristic across the four formless concentrations, when one is detached from the defilements of a lower stage and reborn in a higher stage, they are divided into four according to differences in rebirth. That is, when one has detached from the defilements of the fourth dhyāna and is reborn in the stage above, the stage of boundless space (ākāśānantyāyatana) is established; and so forth, up to when one has detached from the defilements of the stage of nothingness (ākiṃcanyāyatana) and is reborn in the stage above, the stage of neither-perception-nor-non-perception (naivasaṃjñānāsaṃjñāyatana) is established.