We read (in the printed third edition) of Nyanaponika's "Abhidhamma Studies" on page 19/20:
"Phenomenology deals, as the name implies with 'phenomena', that is, with the world of internal and external experience. Ontology, or metaphysics, inquires into the existence and nature of an essence, or ultimate principle, underlying the phenomenal world. … The Abhidhamma doubtlessly belongs to the first of these two divisions of philosophy, that is, to phenomenology."
Bhikkhu Bodhi, in his 'Introduction' to this book changes in page XVI first the "doubtlessly" of Nyanaponika into "rather" and then changes a little bit the meaning of the term 'ontology':
"If, however, we understand ontology in a wider sense as the philosofical discipline concerned with determining what really exists, with discriminating between the real and the apparent …"
And then BB concludes that "the Abhidhamma is built upon an ontological vision."