Here are two pictures a ball nut. They are designed to prevent anything from getting inside the internal part of the nut where the ball bearings are circulating. On both ends of the ball nut you can see a White ring. They are like big thick washers that are threaded to fit the screw. This is a wiper ring and is designed to collect any debris that is on the outside of the screw and push it off. Think of it as a snow plow. The debris will built up on the wiper ring and as it accumulates, it will simply fall off. I’m not sure what the ring is made of but it feels like it delrin or teflon. They fit snug around the screw and do a great job of keeping debris out.
