In the TPX sample library there are samples for the TPXUSIST exit as well as for the TPXUERR1 with code to set a VT unvailable after receiving a sense code 0805.
Which exit should I use?
This depends on the way TPX is notified by VTAM for this condition.
If the error is reported by a TPXL0085 message then VTAM triggered our RPL ERROR exit and you should use TPXUERR1 exit.
If the error is reported by a TPXL0051 message then VTAM triggered our NOTIFY exit and you should use TPXUSIST exit.
Both exits can co-exist handling both the sense 0805 condition.