PEI is something (I’ve picked the word something on purpose because I was never able to find out what It was other than it sands for Porcelain Enamel Institute) that’s widely confused with Tile Classes ” ASTM C1027-19 (2019 edition)” that only measures “Visual Abrasion of The Glaze”. According to 2019 edition, a Class 3 tile is suitable for light commercial and heavy traffic residential surfaces. Almost, all porcelain tiles are in this range!!! If you really want better porcelain tiles, you should check its density (water absorption) and Mohs level(deep abrasion)