The affix ṇyat comes after a verb that ends in the letter u - long or short, when the sense is that of 'necessity'.,
This debars the affix यत्. Thus लु + ण्यत् = लौ + य = लाव्यं 7.2.115 and (VI. I. 79) 'must be cut'; so also पु + ण्यत् = पाव्यम् 'must be purified'.
When 'necessity' is not indicated we have the form लव्यं 'to be cut' The sutra as given in the original is rather ambiguous; it may be construed to mean, 1st when the word आवश्यक or a word having the sense of this term is in construction as upapada, the affix ण्यत् is employed : or 2ndly when, the sense of the affix is that of 'necessity', the ण्यत् is employed. There are objections to both these interpretations taken separately. For in the first case, though we could form the words अवश्यलाव्यं &c., we could not form the word लाव्य by itself. In the second case, though we could form the single word लाव्यं, we could not form the compound. The best solution is to take the second interpretation and form the compound by the universal rule of Tatpurusha contained in (II. I. 72), which would also regulate the accent.,