The affix atasuc (atas) comes in the sense of 'direction', 'locality' or 'time' after the words 1. dakṣiṇa and 2. uttara,
This debars अस्ताति ॥ The word दक्षिणा can never refer to time, so with regard to it direction and locality are only taken. Thus दक्षिणतो वसति, दक्षिणत आगतः, दक्षिणतोरमणीयम् ॥ उत्तरतोवसति, उत्तरत आगतः, उत्तरतोरमणीयम् ॥ The अ of अतस् is for the sake of differentiating this affix from तस् in sutras like 2.3.30.,