Server Sets in MQTT Engine work like they do in MQTT Transmission. Within a given server set, the module will connect to one of the servers. It will not connect to more than one at a time. This feature was added in part for running Ignition in a redundant environment. Without server sets, MQTT Engine would try to connect to all defined servers. Now with sets, Engine can be configured to connect to any one and not all at the same time.
So with Engine in this configuration, MQTT Engine will not continually try to connect to a redundant backup MQTT Server that isn’t online.