
Explanation

Box 1: AMQP
Use AMQP on field and cloud gateways to take advantage of connection multiplexing across devices.
Box 2: MQTT
MQTT is used on all devices that do not require to connect multiple devices (each with its own per-device credentials) over the same TLS connection.
Box 3: HTTPS
Use HTTPS for devices that cannot support other protocols.
Reference:
https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-protocols