Some larger companies and government entities issue contracts for major purchases and then issue specific purchase orders

Question:

Some larger companies and government entities issue contracts for major purchases and then issue specific purchase orders to their contractors according to the terms of the contract.
The contract can specify prices and payment terms as well as other administrative procedures. How would the use of contracts affect the standard process flow as shown in Figure 6.2? How would it affect the UML class diagram for the purchases and payments process?

FIGURE 6.2: Basic Purchases Activity Model

Place Request Prices and Send Receive Items Payment Purchase Order Availability Record Purchase