The 999 TR3 005010X231 shows that I (information) is an allowable value for ISA15 in the Control Segment appendix of the manual. In the Control Section appendix of the TR3's for some of the 5010 Healthcare transactions (837, 270, etc) , I (information) is not shown as an allowable value in ISA15. Is I an allowable value in ISA15 when generating a TA1 response?
Yes, it is allowable. The Interchange instructions located within the other implementation guides are specific to the creation of the ISA for the transaction set referenced in the guide. The interchange envelope instructions for a healthcare TA1 are located in the 999 TR3 005010X231A1.