Information that should always be returned from the I/O Exit to Z Data Tools

The I/O exit should set any non-zero exit return code as appropriate. The exit return code field is initialized to zero on every call from Z Data Tools to the exit.

The I/O exit can also set an informational, warning, or error message into the message field of the I/O exit control block. You can code your exit to return any number of messages, however Z Data Tools will suppress all informational (RC=0) and warning (RC=4) messages after the first 100. Error messages (RC=8 or higher) are never suppressed. The Z Data Tools message: 
Number of messages issued by exit reached the limit. Remaining non-error (RC < 8)
messages will be suppressed
is issued after 100 non-error messages have been produced. The limit of 100 messages cannot be changed.

The I/O exit can set the warning code to 2 at any time, to tell Z Data Tools to continue processing and not call the I/O exit again. The I/O exit will still be called for termination and, if an open was made, for close. For Z Data Tools to check the warning code, the I/O exit must also set the exit return code to 4 (warning).