What is experience actually made of?
Take any moment of experience and ask what it is composed of. Not what it is about — what it is made of.
Abhidharma is the tradition's answer, worked out in exhaustive detail: the constituents of mind and matter, which mental factors accompany which states of awareness, how one moment conditions the next, how actions leave traces that ripen later, and how the realms of rebirth are structured.
Vasubandhu's Treasury of Abhidharma (Abhidharmakośa), from fourth-century India, is a compression of the Great Exposition school's system into verse, together with the author's own prose commentary — which, famously, sometimes argues against the verses.
It is a work of classification more than of argument, and it is dense with lists. The monastery's edition of it is one of the fifteen volumes on the library shelf.
Every earlier subject has been using this vocabulary. Debates about valid cognition assume an account of what a moment of awareness is. Discussions of the grounds and paths assume the mental factors that mark each stage. Emptiness is argued about in terms of what would have to exist for things to exist independently.
Placing Abhidharma at years eighteen and nineteen means a monk arrives at the systematic account having already used its terms for fifteen years — and can see what the system was doing all along.
Abhidharma includes cosmology: the structure of the worlds, their scale, and the beings in them. Some of it is not a description a modern reader would accept literally, and the tradition itself has argued about how much of it is meant that way — a discussion His Holiness the Dalai Lama has taken up publicly in his conversations with scientists.
That the monastery teaches a fourth-century system and also asks how far it holds is not a contradiction. It is what a curriculum built on debate does with an inherited text.
Back to education and intellectual training, or on to the other treatises: Pramāṇa, Prajñāpāramitā, Madhyamaka, Abhidharma, Vinaya.