Final answer:
In PPOs, participants must use network providers for optimal coverage, unlike fee-for-service systems where costs are based on individual services, or HMOs which pay providers a fixed amount per enrollee.
Step-by-step explanation:
Participants of Preferred Provider Organizations (PPOs) must use one of the "network" providers for the majority of their medical bills to be covered; if a patient uses a non-preferred provider, they may have to pay a substantial fee. This is different from a fee-for-service system where medical care providers are reimbursed for the individual services they provide, or a Health Maintenance Organization (HMO), which pays providers a fixed amount per person enrolled in the plan, regardless of the number of services provided. Both PPOs and HMOs aim to control costs and encourage efficient healthcare delivery, each with a unique set of rules regarding providers and reimbursements.