शाच्छोरन्यतरस्याम्

Short इ is optionally substituted for the final of 1. शा (शो) and 2. छा (छो) before an affix beginning with त् and having an indicatory क् ।
Karika
देवत्रातो गलो ग्राह इति योगे च सद्विधिः ।
मिथस्ते न विभाष्यन्ते गवाक्षः संशितव्रतः ॥

As निशितं or निशातम्, निशितवान् or निशातवान्; अवच्छितं, अवच्छितवान् or अवच्छातं, अवच्छातवान् ॥ The शा always takes the इ when it means a vow : as संशितो ब्राह्मणः = संशितव्रतः ॥ The rule of this sutra thus becomes a vyavasthita vibhasha. Other examples of such vyavasthita vibhasha are to be found in 8.2.56, where त्रात and त्राण past participles are formed with त or ण, but त्रात is only employed in names as देवत्रातः &c, and never त्राण, while in denoting action both forms are valid: similarly by 8.2.21, गल and गर are both formed, but गल alone is used when 'neck' is meant, and गर alone when 'poison' is indicated; optionally both when an action is meant. Similarly by 3.1.143, ग्राह and ग्रह are formed, but ग्राहः is only used when 'a crocodile' is meant, and ग्रहः alone is employed when a 'planet' is indicated. Similarly the addition of the Present Participle affix शतृ and शानच् under 3.2.126, is debarred when इति is added, as हन्ति इति पलायते, वर्षतीति धावति ॥ In all the above examples, the option though taught generically, should be limited to specific cases; as also in the examples गवाक्षः 'a window', and गोऽक्षम् 'cow's eye' 6.1.123 and संशितव्रतः ॥ In short, we should limit a general vibhasha to a vyavasthita vibhasha, on the maxim व्यवस्थितविभाषा विज्ञानात् सिद्धम् ॥