User can register its device with MQTT protocol, which is a lightweight messaging protocol mostly used for small sensors and mobile devices. As default, if user changes protocol from "HTTP" to "MQTT" protocol, "JSON" remains as default data format.
User also can choose "UL2.0"(UltraLight 2.0) dataformat in combination with "MQTT" protocol as shown in the following picture
Other data formats are not compliant with "MQTT" protocol, as example user can try pick "LWM2M" data format with "MQTT"
After selecting it, this pick will be rejected with appropriate popup message that says this data format is not coherent with "MQTT" protocol
So user can only use "JSON" and "UL2.0" as a data format with "MQTT" protocol, same as the case with HTTP protocol. Now user can continue with creation of his device as described in device registration