Prusa offers two types of steel sheet beds for their Prusa i3 MK3 series 3D printers: smooth PEI and powder coated PEI. The smooth PEI version is the default, but you can get the powder-coated PEI if you pay extra. It's not cheap but has one or two handy features: it works great for bigger prints, and you can print PET-G on it without any separation layers (stick glue and similar).