What is the business scope of invoicing ultrasonic heat meter?

Ultrasonic equipment, accessories sales and maintenance services. Ultrasonic heat meter is an instrument that uses ultrasonic method to measure flow and show the heat energy released or absorbed by water flow through heat exchange system. When issuing invoices, its business scope includes sales and maintenance services of ultrasonic equipment and accessories. An invoice is the original proof of the cost, expense or income incurred.