大乘入道次第開決 1.59

1.59

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.