Why write your own terminal error program?

There are some situations in which CICS® may try to send a message to an input-only terminal; for example, an ‘invalid transaction ID’ message, or a message wrongly sent by an application program. You should provide a terminal error program to reroute these messages to a system destination such as CSMT or CSTL or other destinations, by means of transient data or interval control facilities.
There could be application-related activity to be carried out when a terminal error occurs. For example, if a message is not delivered to a terminal because of an error condition, it may be necessary to notify applications that the message needs to be redirected.
Not all errors represent communication-system failures; for example, SAM end-of-data conditions.