z/OS MVS Programming: JES Common Coupling Services
SA23-1387-00
Avoid using services that can cause an MVS wait.
If you add an extent to the message packet or alter the length
of the message update, use XIT01_MAX_ADD in IXZ$XPL to reflect
the message size change. XIT01_MAX_ADD contains the maximum
number of bytes that can still be added to a message packet. Be certain
to decrease its size as the message packet grows.