The 'er' vowel is historical back unrounded vowel of unspecified height which in modern Dai Ahom is pronounced as [ə] or [ɤ]

Examples
𑜏𑜢𑜤𑜈𑜫 (ser) - Tiger
𑜉𑜢𑜤𑜂𑜫: (mérng) - Country
𑜎𑜢𑜤𑜄𑜫; (lērt) - Blood

and so on....