Account, Plan, and Purchase are independent resources. An Account is your run-of-the-mill account resource. A Plan contains the details of the current paid plan. A Plan may be created by using Apple's In-App Purchase and submitting the receipt with the Plan template. A Purchase contains the details necessary to perform an In-App Purchase in the Mac client.
Clients will likely never need a Plan or Purchase independently from an Account. It's burdensome to request an Account, Plan, and Purchases in order