अत्रानुनासिकः पूर्वस्य तु वा

This is an adhikara sutra. For the letter which stands before that letter for which रु has been substituted, there is substituted a nasal vowel, in this division of Grammar, where रु is the subject of discussion.

Thus sutra 8.3.5 says \In the place of the final of सम्, there is रु when सुट् augment follows\. The vowel अ of सम् becomes nasal; as सँस्कर्त्ता, सँस्कर्त्तुम्, सँस्कर्त्तव्यम् ॥ Why have we used the word अत्र in the sutra? It means \here i. e. in this division where रु is the subject of discussion\. Objection:- This object would have been gained, without using the word अत्र; since it is an adhikara sutra and would apply to रु ॥ Answer:- No. Here रु is taught in connection with nasal, therefore, the rule would not apply to any other context. For had अत्र not been used, we could not have known the extent of the jurisdiction (adhikara) of this nasal; and we might have applied the rule of nasality to those beyond the jurisdiction of रु, such as 8.3.13. The employment of अत्र prevents this doubt and shows that रु and nasality are co-extensive.,