法華義疏 5.245

5.245

「止住其中」者，五道眾生樂居不捨故云止住。亦則是四十居止：八大地獄與餓鬼、畜生為十，四天下、六欲天復為十，欲界合二十；色界有十六處，無色四處，合四十也。

Regarding "dwelling therein": the sentient beings of the five destinies take pleasure in dwelling there without renouncing it, hence it says "dwelling." This also refers to the forty dwelling places: the eight great hells together with hungry ghosts and animals make ten; the four continents and six desire heavens make another ten; in the desire realm these combine to twenty; the form realm has sixteen places, and the formless realm has four places, combining to forty.