3
"18.1.4 All sets of numbers displayed on a bike must match each other and must be the correct number for the rider. A bike may not have more than one rider number at anytime while on the track proper."
This rule was put in the book this year after several incidents where the rider was improperly identified while on track, specifically a Turn 12 crash where the side number was called in instead of the front plate. Anytime includes practice. I'm getting this out now instead of explaining it on Pit Lane.