大乘百法明門論開宗義決 1.177

1.177

身在上地將生下時等者。此顯上地起下潤生俱生煩惱。謂從上地將生下時。先須起愛潤下地業然始下生。經說非愛不受生故。既潤下業而起下愛。故知上地起上潤生俱生煩惱。

As for "when a body in a higher stage is about to be born in a lower [stage], etc.": this reveals the innate afflictions that moisten rebirth of a lower [stage] arising in a higher stage. That is, when one is about to be born into a lower stage from a higher stage, one must first generate craving (tṛṣṇā) to moisten the karma of the lower stage, and only then is one born below. The sūtra states that without craving, rebirth is not received. Since one moistens lower karma and gives rise to lower craving, we know that innate afflictions moistening rebirth arise in a higher stage.