विभाषा चत्वारिंशत्प्रभृतौ सर्वेषाम्

Operational rule vidhi

The above substitution is the case of all (द्वि , त्रि and अष्टन्) is optional when the word चत्वारिंशत् and the numerals which follow it are the second member.

Word by word (padaccheda)

Substitute / term prathamā · ekavacanaRight context चत्वारिंशत्- प्रभृतौ saptamī · ekavacanaTarget सर्वेषाम् ṣaṣṭhī · bahuvacana

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

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Full reading

चत्वारिंशत्-प्रभृतौ सर्वेषाम्

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)

सर्वेषाम् → / चत्वारिंशत्- प्रभृतौ optional

Replace sarveṣām with vibhāṣā when catvāriṃśat-prabhṛtau follows, optionally. Inherited words (anuvṛtti) may add further conditions.

Commentary

The above substitution is the case of all (द्वि , त्रि and अष्टन्) is optional when the word चत्वारिंशत् and the numerals which follow it are the second member.
Thus द्विचत्वारिंशत् or द्वाचत्वारिंशत्, त्रिपञ्चाशत् or त्र्ययःपञ्चाशत्, अष्टपञ्चाशत् or अष्टापञ्चाशत् ॥ This also before hundred : as, द्विशतम् and अष्टशतम्, त्रिशतम् ॥

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