大毘盧遮那成佛經疏 10.64

10.64

次諸奉教者說此真言，此謂專在本尊之側，承命往來隨有所作者也。亦同上諸金剛，入大金剛無勝三昧而說真言，此一切諸部奉教同用此真言也。

Next, the various kāṅkaras speak this mantra. This refers to those who are exclusively at the side of the principal deity, receiving commands to go and come and carry out whatever is to be done. Likewise identical to the vajradharas above, they enter the great vajra invincible samādhi and speak the mantra; all the kāṅkaras of all families together use this mantra.