Thanks for the response @riturrioz -
This relates to the Homie Convention implemented in the MQTT binding. I have a non-retained item which can be used for momentary buttons. Most of the details are in the post about my Polyglot binding.
Essentially, I have a PlayPause button which is bidirectional. I want Openhab to know when the remote side sends PlayPause (this is captured in the trigger) - but I also want to send PlayPause to the remote device from OpenHab.