The carrier arm is the part that ride up and down on the guide rails. It is the "arm" that the bearing roller bolts to, and that turns in the "Y" guide slot. The hub rides in between the rails. It is the HUB that keeps everything centralized between the rails. The areas on the hub which is the biggest concern is the diameter of the hub, and the top flanges that ride on the top of the guide rails. The 'underside' flanges are less of a concern...but still can rust and cause galling on the underside of the guide rails. But to avoid problems, all parts need to be smooth, and well lubed, for easy sliding.
In your photo, your rails showed 'galling' and 'scratching'. Two different conditions. Galling occurs when two parts stick together over time and then move. When they move one part that was stuck, pulls or displaces metal away from one area of another part to deposit it on another area of that same part. Scratches are when one part is rough and that roughness slides over the other part to "rough" it up.