Jātaka (Lanna chadok) are the birth-stories: tales of the Buddha's previous lives as the Bodhisatta, working his way through the perfections across some five hundred existences. In the Pali canon they sit in the Khuddaka-nikāya, grouped by the number of verses each tale carries — the nipāta, which surfaces directly in the titles here (Wisati nibat, the twenty-verse group; Panca nibat; Sap tingsa nibat). But the Lanna jātaka corpus is wider than the canon. Its living heart is the paññāsa jātaka, the extra-canonical "fifty births" composed in mainland Southeast Asia, and above all the Mahāchat — the Vessantara, the Great Birth, whose annual recitation (Thet Mahachat) is one of the central preaching festivals of the northern year.
So while this is not a wichaa genre, it is not inert scripture either. These were the preacher's working repertoire, copied to be read aloud; the local verse retellings that fill out the collection (Along this and that) are Lanna literature as much as they are Buddhist canon.
Jātaka is one of the largest genres in the collection — over 1,600 witnesses — and, like the canonical texts it sits beside, it is written almost entirely in Tham Lanna (94%) on palm-leaf (88%). Paper barely registers here: a jātaka is a durable sermon-text, copied into the same canonical format as the scripture it accompanies, not a working almanac to be opened flat.
On paper the collection looks like a Phrae–Lampang tradition (Phrae 27%, Lampang 22%), but that shape is deceptive and worth reading carefully — see the first note below.