AMQPHelpers.Adapter.register_return_handler

You're seeing just the callback register_return_handler, go back to AMQPHelpers.Adapter module for more information.
Link to this callback

register_return_handler(channel, handler)

Specs

register_return_handler(channel :: AMQP.Channel.t(), handler :: pid()) :: :ok

Registers a handler to deal with returned messages.

See AMQP.Basic.return/2.