Refers to the party placing an order, especially in business or procurement settings. Often used in contrast with 受注者 (order receiver).
A contract was concluded between the orderer and the order receiver.
We changed the specifications according to the customer's request.
注文者 is a more general term for someone placing an order, while 発注者 is more formal and business-oriented.
Compound of 発注 (order placement) + 者 (person).