The affix aṇ -- prāgdīvyato'ṇ [[4.1.83]], is added to the name of a deity when something is to be spoken of as his.,
सा = 'after a word in the Ist case in construction' अस्य 'in the sense of a sixth case'. देवता 'if the word in the first case is the name of a deity'. Thus इन्द्रो देवताऽस्य = ऐन्द्रम् 'oblation sacred to Indra'. Similarly आदित्यम् 'oblation belonging to Aditya'. So also बार्हस्पत्यम्, प्राजापत्यम् ॥ Why do we say \after the name of a deity\? Observe, कन्यादेवताऽस्य, no affix is added to कन्या ॥ In the sentences ऐन्द्रोमन्त्राः and आग्नेयो वै ब्रह्मणोदेवतयेति the affix is added by analogy.
The whole phrase साऽस्य देवता governs up to sutra 4.2.35. Though the word स was understood in this sutra by anuvritti from sutra 4.2.21, its repetition is for the sake of indicating that the anuvritti of संज्ञा does not extend to it. The affix ordained by this sutra has the force of indicating 'lord of oblations like Purodasa &c offered in a sacrifice'.,