A message switch occurs when a user wants to SEND a message to another user. The basic format of a message switch is the destination LTERM NAME, followed by a blank and the message text.
A program-to-program switch or program-to-program message switch is a program that is already executing that requests a NEW transaction be put on the IMS message queues for STANDARD scheduling and execution.
This new transaction can do the following: Continue the processing of the first transaction and respond to the originating logical device
Process as an INDEPENDENT transaction unit of work.