Amqpx.Basic.return

You're seeing just the function return, go back to Amqpx.Basic module for more information.
Link to this function

return(channel, return_handler_pid)

Specs

return(Amqpx.Channel.t(), pid()) :: :ok

Registers a handler to deal with returned messages.

The registered process will receive {:basic_return, payload, meta} tuples.