रात्रेः कृति विभाषा

Operational rule vidhi

The word रात्रि optionally takes मुम् augment before a word formed by कृत् affix.

Word by word (padaccheda)

Target रात्रेः ṣaṣṭhī · ekavacanaRight context saptamī · ekavacanaSubstitute / term prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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").
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

रात्रेः

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)

रात्रेः → / optional

Replace rātreḥ with vibhāṣā when kṛti follows, optionally. Inherited words (anuvṛtti) may add further conditions.

Commentary

The word रात्रि optionally takes मुम् augment before a word formed by कृत् affix.
As रात्रिचरः or रात्रिंचरः, रात्रिमटः or रात्र्यटः ॥ This is an aprapta-vibhasha. The augment is compulsory before a krit-affix having an indicatory ख, As, रात्रिंमन्यः ॥

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