The affix yat comes after a root which ends in a labial letter preceded by a short a,
This debars ण्यत्; thus शप् + यत् =शप्यं 'to be sworn'; लभ् + यत् = लभ्यम् 'to be acquired'.
Why do we say 'which ends in a labial'? Because if a root ends in any other consonant, though preceded by a short अ, the root will take ण्यत्; thus पच् + ण्यत् = पाक्यम् 'what is to be cooked'; वाक्यम् 'speech'.
Why do we say 'when preceded by a short अ'? When preceded by any other vowel, it will take ण्यत्; thus कुप् + ण्यत् = कोप्यम् 'to be angry'; so also गोप्यम् 'to be concealed'.
The word अत् is used in the sutra in the original. The force of त् here is, by (I. I. 70), that short अ having one matra or prosodial length is to be taken and not long आ. Therefore, after the root आप् 'to obtain', we have ण्यत् and not यत्; thus आप्यम् 'to be reached' i. e. 'attainable'.,