Outpatient Prospective Payment System – In response to rapidly growing Medicare expenditures for outpatient services and large co-payments being made by Medicare beneficiaries, Congress mandated that the Centers for Medicare and Medicaid Services (CMS) develop a Hospital Outpatient Prospective Payment System (HOPPS) and reduce beneficiary co-payments.
Further reading
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2988668/