en_qos_choices
This enum has the following members:
0- QOS_AT_MOST_ONCE
1- QOS_AT_LEAST_ONCE
2- QOS_EXACTLY_ONCE
Hivemq.com has an excellent explanation of the MQTT quality of service levels: http://www.hivemq.com/blog/mqtt-essentials-part-6-mqtt-quality-of-service-levels.