सात्पदाद्योः

Operational rule vidhi

The ष् substitution does not take place in the affix सात् and for that स् which stands at the beginning of a word.

Word by word (padaccheda)

Target सात्- पदाद्योः ṣaṣṭhī · dvivacana

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

Full reading

सात्-पदाद्योः

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

Commentary

The ष् substitution does not take place in the affix सात् and for that स् which stands at the beginning of a word.
Thus सात् here is the affix ordained by 5.4.52, and it would have been changed to ष by 8.3.59. The present sutra prevents that. The Padadi स refers to those verbs which in the Dhatupatha are taught with a ष, and for which a स is substituted. This also is an exception to 8.3.59. Thus अग्निसात् । दधिसात् । मधुसात् ॥ So also with Padadi स as दधि सिञ्चति, मधु सिञ्चति ॥

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