तत्र कुशलः पथः

Operational rule vidhi

The affix वुन् comes in the sense of 'versed therein or skilled therein' after the word पथिन् being in the Locative -7th case in construction.

Word by word (padaccheda)

Indeclinable तत्र avyayaSubstitute / term कुशलः prathamā · ekavacanaLeft context पथः pañcamī · 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.
Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.

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Full reading

तत्र कुशलः पथः

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

Commentary

The affix वुन् comes in the sense of 'versed therein or skilled therein' after the word पथिन् being in the Locative -7th case in construction.
Thus पथि कुशलः = पथकः (पथिन् + वुन् = पथ् + वुन् 6.4.144 = पथ् + अक् 7.1.1.

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