Request and reply is an integration pattern that allows a system to send a message to another system and wait for a response. This pattern is suitable for scenarios where the sender needs to receive an immediate confirmation or acknowledgment from the receiver. In this case, B2C Commerce needs to receive the order number and status from the ERP system after sending the order details. Reference:
https://architect.salesforce.com/design/integration-patterns-and-practices/request-and-reply/
https://documentation.b2c.commercecloud.salesforce.com/DOC1/topic/com.demandware.dochelp/OrderManagement/OrderManagementOverview.html