Do you see a pattern here? Even though these models serve different departments in the enterprise, they in fact model exactly the same thing—a sales transaction. The only difference is that in one case we are the vendor, and in the other case we are the customer. The structure of both transactions is the same.
Note that the terms “customer” and “vendor” do not represent fundamental “things of significance” to the enterprise. Rather they describe roles. The underlying reality is the Person or Organization that is doing the buying or selling.
Figure 15-3: Contracts
In other words, a Person or an Organization is not inherently a customer ...