The RequiredOrderSubtotal component, which appears in the Order Subtotal stage of an order processing pipeline, checks the _oadjust_subtotal member in the OrderForm to ensure that this value is not NULL. If NULL, RequiredOrderSubtotal generates an error.
If you are not running the Pipeline Editor in expert mode, the RequiredOrderSubtotal component is automatically inserted into the Order Subtotal stage, but is not displayed in the Pipeline Editor user interface. This component appears in the user interface only if you run the Pipeline Editor in expert mode.