ऊर्णोतेर्विभाषा

Operational rule vidhi

Before an इट् beginning substituted-Aorist of the परस्मैपद there is optional वृद्धि of the vowel of ऊर्णु.

Word by word (padaccheda)

Target ऊर्णोतेः ṣaṣṭhī · ekavacanaSubstitute / term prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

ऊर्णोतेः

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)

ऊर्णोतेः → optional

Replace ūrṇoteḥ with vibhāṣā, optionally. Inherited words (anuvṛtti) may add further conditions.

Commentary

Before an इट् beginning substituted-Aorist of the परस्मैपद there is optional वृद्धि of the vowel of ऊर्णु।
As प्रौर्णावीत् or प्रौर्णवीत् ॥ This option applies when the सिच् is not treated as ङित् ॥ But after ऊर्णु, सिच् is optionally ङित्, 1.2.6: when it is ङित्, there being neither guna nor vriddhi; we have उवङ् substitution, as प्रौर्णुवीत् 6.4.77.

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