After a word in the Nominative 1st-Case in construction, an affix comes in the sense of 'this is his dwelling place'.,
The word स shows that the word to which the affix is to be added is in the nominative case. अस्य 'his' shows that the meaning of the affix is that of a genitive case. निवासः, shows that the first word in construction must signify a dwelling place. निवास means \a country i. e. where a person lives (nivasanti asmin)\.
स्रुघ्नो निवासोऽस्य = स्रौघ्नः 'a present dweller of Srughna', माथुरः, राष्ट्रियः &c.,