AMQPHelpers.Adapter.register_confirm_handler

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

register_confirm_handler(channel, handler)

Specs

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

Register a handler for confirms on channel.

See AMQP.Confirm.register_handler/2.