"Concerning this statement, “Form is empty of form,” there are also three aspects of form—imagined form, imputed form, and the form of the true nature.

That which is the form of entities apprehended by childish ordinary persons as characteristics suitable to be form and so forth is known as “imagined form.”

Precisely that, in whatever aspect it becomes an object of consciousness appearing as an external entity, is known as “imputed form.”

That which is free from the aspects of both the imagined and the imputed, and is solely the fully established thusness, is known as “the form of the true nature.”

That which is the fully established, the form of the true nature, is empty of the characteristics of existence as imagined form and so forth, and also empty of form that appears in the aspect of an object imputed as form, so it is known as “empty.”

Thus it is explained. But you may have doubts, wondering, “Does that which is the form of the true nature, empty of imagined and imputed form, have some other characteristic of form? Why is it even known as ‘form’?”

Therefore it is explained, “That which is the emptiness of form is also not form.” Precisely that which is empty of imagined and imputed form is the characteristic of the fully established. That form of the true nature is not the quintessence of form, because it is in all aspects isolated from the aspect of form."

Vasubandhu, Vast Explication (Bṛhaṭṭīkā)