2.3.62 चतुर्थ्यर्थे बहुलं छन्दसि

Word by word (padaccheda)

Right context चतुर्थी- अर्थे saptamī · ekavacanaSubstitute / term बहुलम् prathamā · ekavacanaRight context छन्दसि saptamī · ekavacana

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.

Full reading

चतुर्थी-अर्थे बहुलम् छन्दसि

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

Commentary

In the छन्दस् the 6th-Case-affix is used diversely with the force of the 4th case-affix.
Ex. पुरुषमृगश्चन्द्रमसः or चन्द्रमसे 'to the moon, a male deer.' गोधा कालका दार्वा घाटस्ते वनस्पतीनाम् or वनस्पतिभ्यः, to you lords of the forest, are lizard, kalaka bird, &c. So also वायुरस्मा उपा मन्थत् (R. Ved X. 136.7).

Why do we say 'diversely?' Observe कृष्णोरात्र्यै, हिमवती हस्ती.

Vart:- In the Veda, the 4th case-affix is employed in the sense of the sixth : as या खर्वेण पिबति तस्यै खर्वोजायते 'who soever woman drinks with a woman in her courses, gets herself in menstrua. So also या दतो धावति तस्यै श्यावदन् 'who cleans her teeth, her teeth become black.' या नखानि कृन्तति तस्यै कुनखः 'who pairs her nails, her nails become ugly.' So on, in याऽङ्क्तेतस्यै काणः याऽभ्यङ्क्तेतस्यै दुश्चर्मा, या केशान् प्रलिखति तस्यै खलति ॥ अहल्यायै जारः (T.S. 2.5.1.7.)

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