MQTT message and "datr" JSON item

When sending a message via the MQTT server, there is a field I can add to the JSON object called “datr – the LoRa datarate identifier”. How is this field supposed to be used? Do I have to specify it whenever I send a message? What happens if I don’t specify it? If I have to specify it, how can I get the data rate at the time I need to send a message?

Thanks,
Dave.