This is the higher-doctrine basket — the Abhidhammapiṭaka, the systematic, analytical distillation of the teaching into its bare categories of mind and matter. At 273 witnesses it is the smallest of the three canonical piṭakas here, and by some distance the most technical and most scholastic — the corpus of the specialist.
The most telling pattern is which Abhidhamma text the north actually copied. It is not, overwhelmingly, the seven canonical books in the raw — it is the great commentary on them, the Aṭṭhasālinī (Aṭṭhasālinī, Nisai athasalini, Atthasālinī nāma abhidhammasaṅgiṇī), which recurs again and again, often in multiple phuk (fascicles: phuk si, phuk hok). Around it sit the Saṅgaha digests (Nisai sangkaha, Piccalana ālambana saṅgaha) — the Abhidhammatthasaṅgaha, the standard textbook that made the system teachable. The Lanna Abhidhamma is thus a corpus approached through its exegesis and its compendia, not through the primary canon — you reach the higher doctrine by way of the commentary that explains it.
Its physical profile is canonical — 95% palm-leaf (260 of 273), 220 dated, spanning 1497–1946 (its oldest witness rivals the sutta's for the earliest in the collection). Two figures mark it as the scholar's basket. First, language: 142 monolingual Pali against 104 Pali-and-Lan Na — the only canonical sub-genre where untranslated Pali is the majority, the signature of a text read by those who needed no vernacular crutch. Second, concentration: Wat Sung Men holds 152 of 273 (56%) — the highest keystone-library share of the three piṭakas, fitting for the most rarefied material.