I don’t know what you call it. I tried Googling things like “reinforcement fabric pants hem,” but no dice. All I know is that there is an extra strip of “reinforcement” fabric inside the rear hem of the pants of one of my custom suits. It’s obviously designed to prevent any destructive scuffing and chafing that can occur from the back of my shoes rubbing up against the inside of my pants. It’s subtle and completely invisible from the outside. Whatever this feature is called, I like it.