Do you mean "4.10 Request / Response"?
I have a program that runs somewhere in the cloud that monitors my car and records observed state in a local database while also publishing those observations via mqtt. I have another program that connects to the same mqtt broker and watches those observations to store into a local database. I built a sync protocol over rpc over mqtt to ensure all of the records that have been recorded in the cloud application's database is replicated into my local (laptop) database.
I also have various controls over this protocol. For example, I can adjust my heat and steering wheel heater and cabin temperature from a simple rpc command that doesn't have credentials for my car and doesn't have any idea where the actual control application lives. The response tells me whether it worked or not.