If 'No' is selected it may cause the MQTT message to be echoed back.
In most cases this is a bad scenario and should be avoided.
Select the Topic('s) Domoticz will use to publish outgoing messages. Flat - publish outgoing messages on topic {domoticz/out}. Floor/Room - publish outgoing messages on topic {domoticz/out}/{$floorplan name}/{$plan name}. Flat + Floor/Room - Use both Flat and Floor/Room topic schemes. Index - publish outgoing messages on topic {domoticz/out}/{$idx}. (with or without Retain bit) Name - publish outgoing messages on topic {domoticz/out}/{$name}. (with or without Retain bit) None - disable outgoing messages.
Note that Floor/Room only reports sensor updates for sensors that are placed on a floorplan/plan.