The source code for Exit TPXERR1 we provide in member 'hlq.CB0VSRC(TPXERR2)' is checking the RPL for sense x'0805' on an Application Start Request. If this sense code is replied, the virtual terminal is made unavailable for any future Session Start Requests.
While this is a useful function you might consider to expand functionality of this exit to check for Sense Code x'0821', too. But we cannot recommend this. As x'0821' is a pretty common Sense Code you might make a large number of virtual terminals unavailabe and therefore might need more virtual terminals for TPX.