You’ve found the emonhub setting.
I note you say client i.e. the system that is subscribing or publishing and that system will need the broker settings (the broker does not care).
The emonpi has it’s own broker at present if you use the prebuilt image I believe. You can check - on the admin page it will list the MQTT Host (it actually means broker) IP. If this is the same as the emoncms IP it is the emonpi broker.
This value is set in the settings.php file and that address is used for both publishing and subscribing.
I’d suggest disabling the mosquitto broker on the emonpi if you are not going to use it.