सनः क्तिचि लोपश्चास्यान्यतरस्याम्

Operational rule vidhi

The long आ is optionally substituted for the final of सन् before the affix क्तिच् and there is also optional elision of the nasal.

Word by word (padaccheda)

Target सनः ṣaṣṭhī · ekavacanaRight context क्तिचि saptamī · ekavacanaSubstitute / term prathamā · ekavacanaIndeclinable च avyayaTarget अस्य ṣaṣṭhī · ekavacanaRight context अन्यतरस्याम् saptamī · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

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Full reading

सनः क्तिचि च अस्य अन्यतरस्याम्

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

As a rewrite rule (sketch from case endings)

सनः अस्य → / क्तिचि अन्यतरस्याम्

Replace sanaḥ asya with lopaḥ when ktici anyatarasyām follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

The long आ is optionally substituted for the final of सन् before the affix क्तिच् and there is also optional elision of the nasal.
Thus we have three forms सातिः꣡, सन्तिः꣡ and सतिः꣡ ॥ The word अन्यतरस्याम् 'optionally' has been employed in the sutra for the sake of clearness only; for the विभाषा of 6.4.43 could have been read into it by anuvritti. Lest any one should doubt, that the anuvritti of विभाषा had ceased with the last aphorism, this word अन्यतरस्याम् is employed here.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.