'population equivalents'. We use it to design sewage plant capacity. But you're right, it doesn't seem to be documented in osm and it looks like it's barely used at all. For now I'll delete it while I dig out some 'cubic meter/day' numbers in stead. I believe m3/d is the usual unit when tagging wastewater facilities in osm, but I'll be sure to check up on it. Thanks for letting me know. 