तत्र तस्येव

Operational rule vidhi

The affix वति (वत्) also comes in the sense of 'like what is therein or thereof'.

Word by word (padaccheda)

Indeclinable तत्र avyayaTarget तस्य ṣaṣṭhī · ekavacanaIndeclinable इव avyaya

Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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Full reading

तत्र तस्य इव

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The affix वति (वत्) also comes in the sense of 'like what is therein or thereof'.
As मथुरायामिव = मथुरावत् 'like that in Mathura'. as मथुरावत् स्रुघ्ने प्राकारः, पाटलिपुत्रवत् साकेते परिखा; देवदत्तस्येव देवदत्तवद् यज्ञदत्तस्य गावः &c.

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