What is the criteria or conditions in TPX to make the Virtual Terminals available when the TPXUERR1 exit is in place and the flag is set that will cause the Virtual Terminals to be checked at the INQUIRE CHECKING INTERVAL?
The sense code, for example 0805xxx is generated by VTAM. TPX simply intercepts this and the TPXUERR1 exit is driven and marks the Virtual Terminal unavailable. TPX then uses the INQUIRE INTERVAL time set to INQUIRE from VTAM the status of this Virtual Terminal. When VTAM identifies it is released/available; TPX resets the Virtual Terminal flag to available for the unavailable Virtual Terminal.
In summary TPX does nothing to make the Virtual Terminals available, it is VTAM that is doing this under the covers.