भवतेरः

Operational rule vidhi

When followed by लिट्-लकार, the अभ्यास of the verb root भू is converted to अकार.

Word by word (padaccheda)

Target भवतेः ṣaṣṭhī · ekavacanaSubstitute / term अः 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

भवतेः अः

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)

भवतेः → अः

Replace bhavateḥ with aḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

अ is substituted for the vowel of the reduplicate of भू 'to be' in the लिट् (Perfect Tense).
As बभूव, बभूवतुः, बभूवुः, बभूवे ॥ Why do we say 'in the Perfect'? Observe बभूषति and बोभूयते ॥

The word भवतेः is shown in the sutra in the Active Voice, therefore the rule will not apply to Passive and Reflexive forms; as अनुबुभूवे कम्बलो देवदत्तेन ॥ This opinion of Kasika, however, is not endorsed by later Grammarians.

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