3.1.91 opens a long stretch of the Aṣṭādhyāyī with a one-word scope: everything that follows, until stated otherwise, is prescribed after a root. Inside that stretch, 3.1.93 says the rest of the sentence in three words: an affix that comes after a root and is not तिङ् gets the saṃjñā कृत्. Not "one of these forty affixes" — whatever is left once the तिङ् endings (the eighteen personal verb-endings, त्/सि/मि… — 3.4.78 , walked in full on page 11) are excluded.
This corpus's own kṛt table, C.krt(), carries affixes across groups — the working slice of a class the Aṣṭādhyāyī itself never bothers to count, because it is not a list, it is everything धातोः अतिङ् from 3.1.91 to the end of the third adhyāya. Page 17 and page 18 already take a double-digit share of that number — the participles, and क्त्वा/ल्यप्/तुमुन् — in real depth. This page does not repeat either. It shows the one mechanism that makes all of them predictable instead of memorised.