It is difficult to generalize about MCOs because they take many different forms. Although most medical care practitioners are familiar with the notions of whether MCOs are open panel or closed panel, an HMO or a PPO, these descriptions do not convey the variability of MCO organization. This variability makes it very difficult for physicians or the public to know with certainty what the MCO provides and how to best deal with it. The variability also causes legal problems for MCOs and their medical care practitioners because administrative practices that might be legal under one organizational structure are illegal in others. For example, the rules governing physician incentives under Medicare prohibit practices that are legal for private pay patients. [Medicare and Medicaid Programs, Requirements for Physician Incentive Plans in Prepaid Health Care Organizations, 61 Fed. Reg. 69,034 (Dec. 31, 1996).]