Only has to be legible, visible from the out side or accessible without the use of tools (e.g. Inside the battery compartment. I printed mine on a double of stickers and put one on top of the shell along with the 'if found call xxxxxx, Reward' sticker. Put the other on the battery just in case they got separated. When I registered them for commercial use, I put the N numbers on the outside of each rear arm.