ExBinance.Private.cancel_order_by_client_order_id

You're seeing just the function cancel_order_by_client_order_id, go back to ExBinance.Private module for more information.
Link to this function

cancel_order_by_client_order_id(symbol, client_order_id, credentials)

See ExBinance.Rest.CancelOrder.cancel_order_by_client_order_id/3.