Re: X12 864 or X12 997, which will be the formal use for customized error message?

Perhaps you can manipulate it, but why?  The 997 simply acknowledges that you received the EDI transmission and Biztalk handles all the details for you.  It is not intended to give any information about whether the contents of the transmission were usable.  The 824 message type exists for just that purpose.  If you intercept and modify the 997, then every Biztalk upgrade becomes much harder.  If your vendor changes their EDI software, then you might have problems.  Save yourself all these headaches and let Biztalk send the 997 and then generate and transmit the 824 in an orchestration that receives the 210.  If you use the EDI protocol the way it was intended, you will  have far fewer headaches supporting it.