ससजुषो रुः

Operational rule vidhi

The सकार at end of a पद, and the षकार at end of the सजुष् पद get a रुँ-आदेश .

Word by word (padaccheda)

Target ससजुषोः ṣaṣṭhī · dvivacanaSubstitute / term रुः prathamā · ekavacana

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

ससजुषोः रुः

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)

ससजुषोः → रुः

Replace sasajuṣoḥ with ruḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

For the final स् and for the ष् of ससजुष् is substituted रु , at the end of a word.
Thus अग्निरत्र, वायुरत्र ॥ So also सजूर्ऋषिभिः, सजूर्देवेभिः ॥ सजुष् is derived from जुष् with the affix क्विप् and the preposition सह, which is changed to स in Bahuvrihi. The lengthening takes place by 8.2.76. and the word means सप्रीतिः ॥ The रु is र्, but it should be distinguished from it. This secondary र् (or रु) undergoes a distinct and separate Sandhi change from that of the primary र् ॥

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