If you look on the downtube, you’ll see some green/blue sticker things with a number on it. Those were my past registrations, but it’s only good for a year each time. You can register it for this year through UCB PD for like $10. It’s not required but it is supposed to be helpful if the bike is stolen. Also, if you turn the bike upside down, on the bottom there is a number stamped into the metal (where the pedals go into the frame).If you know that number (haha)you’d be able to give that to the police as a way to ID your bike if it is stolen. If you report a bike with that number as stolen, bike shops are supposed to run a cross check on it before selling a used bike to a new owner. So, it might be a good precaution for the future to at least write that number down.