金剛經纂要刊定記 3.45

3.45

現跡者。所現之化跡也。所表者，諸佛所為必不率爾，皆以事相表內身心。如說《如來藏經》舉身放光，光中現華，華萎見佛。遂阿難問佛，佛為說之。如《華嚴》中說：「佛菩薩說天說雲須彌山大海等」，皆有所表。斯皆事相為能表，法為所表，以不徒然，故云必也。

"Manifested traces" refers to the manifested traces of transformation. "What is expressed": whatever the buddhas do is never haphazard, but always uses external phenomena to express the internal body and mind. For instance, when expounding the Tathāgatagarbha Sūtra, his entire body emitted light, within the light flowers appeared, and when the flowers withered, buddhas were seen. Thereupon Ānanda questioned the Buddha, and the Buddha expounded it for him. Just as in the Avataṃsaka Sūtra, which speaks of "buddhas and bodhisattvas speaking of the heavens, clouds, Mount Sumeru, the great ocean, and so forth," all have something expressed. In all these cases, external phenomena serve as what can express, and the Dharma serves as what is expressed; since it is not in vain, it is said to be "must."