I use informix on Solaris + Soltice Backup but I think it's the same way,
ixbar file , it like a notepad that note for database backed up data so if database backup some kind of data that's in the tape it can relate with ixbar file ONLY SOME LINE in the file , so you must find and choose the line of ixbar file to relate with your tape , I recommend you open and edit you ixbar file choose the line that it backed up data in to the tape , the exact sequance line of the ixbar file