The final portion of a word, beginning with the last among the vowels in the word, is called ṭi,
This sutra defines टि. It is that portion of a word which is included between the last letter and the nearest vowel. Thus in the word अग्निचित् the portion इत् is टि; so also in सामसुत् the portion उत्. The word टि occurs in sutra 3.4.79.
The word अचः in the sutra is in the genitive case, which has the force of nirdharana or specification; and though it is in the singular number, it represents class name, meaning 'among the vowels.',