How do I decide what class of device to use for the BTM511?

The class of device (COD) format is defined by the Bluetooth SIG on the following webpage:

https://www.bluetooth.org/en-us/specification/assigned-numbers/baseband

The following site can be useful when generating a COD:

http://bluetooth-pentest.narod.ru/software/bluetooth_class_of_device-se…

A good way of understanding COD is to perform an at+btin command and discover the COD of devices around you. For example if you are working on a Bluetooth speaker, use the command at+btin to find out what COD other Bluetooth speakers are using and then refer back to the web pages linked to above.