[I]t is extremely important to notice that a connection was made early on between the idea of immeasurability and emptiness.

In the Perfection of Insight [Wisdom] in Eight Thousand Lines (i.e., the Aṣṭasāhasrikā Prajnāpāramitā-sūtra), the disciple Subhūti asks the Buddha how a bodhisattva in training can recognize or “apperceive” the perfection of insight and is told that this is done through a series of thoughts which are “inclined toward all-knowledge.” Why so?

Because all-knowledge is immeasurable and unlimited. What is immeasurable and unlimited, that is not form, or any other skandha. That is not attainment, or reunion, or getting there; not the path or its fruit; not cognition or consciousness; not genesis, or destruction, or production, or passing away, or stopping, or development, or annihilation. It has not been made by anything, it has not come from anywhere, it does not go to anywhere, it does not stand in any place or spot. On the contrary, it comes to be styled “immeasurable, unlimited.” From the immeasurableness of space is the immeasurableness of all-knowledge.

This passage shows us that the term “immeasurability” belongs to those which indicate the aspect of being without characteristics. It is not intended to make an ontological assertion. Rather, it is intended to indicate the aspect of “positionlessness,” a term with which one may satisfactorily summarize the character of the Prajn͂āpāramitā and Mādhyamika schools.
A Buddhist Kaleidoscope; Michael Pye, The Length of Life of the Tathāgata, Page 167-168