The affix ṭhan and ṭhak comes after the words in the Accusative -2nd case in construction denoting combination in the sense 'who assembles there'.,
The word समवाय means 'combination, collection &c'. The word समवायान् is in the plural number in the sutra, indicating that the svarupa-vidhi (1. 1. 69) does not apply; the affix being applied not only to the word-form समवाय but to its synonyms also. समवैति means coming together. Thus समवायान् समवैति = सामवायिकः 'who assembles in an assembly'. So also सामाजिकः, सामूहिकः, सान्निवेशिकः ॥,