सर्वत्र विभाषा गोः

Operational rule vidhi

In the context of संहिता, an ओकार occurring at the end of the पदसंज्ञक गो शब्द exhibits optional प्रकृतिभाव when followed by a ह्रस्व अकार.

Word by word (padaccheda)

Indeclinable सर्वत्र avyayaSubstitute / term prathamā · ekavacanaTarget गोः ṣaṣṭhī · ekavacana

Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

सर्वत्र गोः

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)

गोः → optional

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

Commentary

After गो the subsequent अ may optionally be retained everywhere, in the Vedas as well as in the secular literature.
Thus गोऽग्रं or गो अग्रं ॥ In the Vedas also अपशवो वा अन्ये गो अश्वेभ्यः, पशवो गोऽश्वाः ॥

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