大乘入道次第開決 1.59

1.59

不被一切有相等者。即約三義釋不動名。功用即是加行異名。前第七地雖亦無相及無煩惱。而有加行行動相不動。未得名不動。今至第八地相行俱不動。故與第七有差別也。

As for "not being [perturbed] by any [signs], etc.": this explains the name "Immovable" in terms of three meanings. "Exertion" (gongyong) is an alternative name for preparation (prayoga). Although the previous seventh bhūmi is also signless and without afflictions, it has preparation: its activity is mobile while its marks are immovable, so it does not yet receive the name "Immovable." Now, on reaching the eighth bhūmi, both marks and activity are immovable; hence it differs from the seventh.