The expectation is that this is one of a DEINIT message, an INIT message or the first message of a NEW transaction whose transaction number is to be set into '*messid'.
When used to send an INIT message, ams_getfreetrans() is used to obtain a free temporary transaction and the MESSYS__INIT is sent to the other task's command queue (using ams_sendinit()).
When used to send a DEINIT, ams_senddeinit() is used to send the MESSYS__DE_INIT message to the other task's command queue.
AMS The Unix ADAM Message System