The content on IDIQ contracts at FAR 16.504(a)(1) states: "The contract must require the Government to order and the contractor to furnish at least a stated minimum quantity of supplies or services. In addition, if ordered, the contractor must furnish any additional quantities, not to exceed the stated maximum." The clause at 52.216-22 (Indefinite Quantity) contains similar language. These are the actual references you are asking about. Of course, contracting officers are free to ask contractors to sign the delivery orders for acknowledgement, but doing so is not required to establish the responsibility of the contractor to fulfill orders as stated in the basic contract.