I have found the following ties to line 2a on 1040 (tax exempt interest): the sum of tax exempt interest (line 8 on 1099-INT) less bond premium (line 13 on 1099-INT). In addition, it includes from 1099-OID: OID on tax exempt obligations (line 11) less acquisition premium (line 6). I find this in detail found in the 1099s received from brokerage companies - combine DIV, INT, and OID 1099s in one file.
