Quotes from Compendium of Abhidharma, Vol. 3
“What is the characteristic of emptiness (śūnyatā)?
It is the non-existence (abhāva) of a certain thing, there. To observe (samanupaśyanā) in this way is emptiness.
Again, it is the existence (bhāva) of another thing, there.
In this way there is real knowledge (yathābhūtajñāna). This is called emptiness perceived by penetration (avatāra). Real knowledge means informed meaning.
What is the meaning of the non-existence of a certain thing, there? It is the non-existence in the aggregates, elements and spheres, of a self or anything pertaining to a self of a permanent, enduring, eternal and immovable nature. This is their emptiness.
What is the meaning of the existence of another thing, there? It is the fact that there is non-self in them. It is the non-existence of self, and the existence of non-self. It is with this meaning that the Blessed One said: ‘Existence is real knowledge of an existing thing; non-existence is real knowledge of a non-existing thing.’"