We only calculate the data that is sent to the listeners who are connected to our server - external servers are not counted. Even the stream to the external server from us is not counted in the data transfer limit.
We have other plans with higher data transfer limit. Or you can buy additional data transfer for the Small plan:
https://www.radioboss.fm/radioboss-cloud/upgrading-your-account/
You don't need to care about data transfer limit if you sent the stream to some other server.
I'd also like to note that there's no such thing as "unlimited" data transfer. It's always limited, and it's noted in fine print. If you actually use a lot of data, they will ask you to pay or leave.