वोपसर्जनस्य

Operational rule vidhi

स is optionally substituted for सह when the compound is a बहुव्रीहि समास.

Word by word (padaccheda)

Indeclinable वा avyayaTarget ṣaṣṭhī · ekavacana

Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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Full reading

वा

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

Commentary

स is optionally substituted for सह when the compound is a बहुव्रीहि समास।
That compound in which all members are secondary-उपसर्जन-is a सर्वोपसर्जन, and the Bahuvrihi is such a compound, because in it all the members are secondary, the Principal being understood. Thus सपुत्रः or सहपुत्रः, सच्छात्रः or सहच्छात्रः ॥ Why do we say of a Bahuvrihi? Observe सहयुध्वा, सहकृत्वा ॥ The substitution does not however take place in a Bahuvrihi like सहकृत्वप्रियः or सहयुद्धप्रियः ॥

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