Having this 12-digit/letter sequence saved ensures you can easily replace it via warranty or prove it is yours if it is returned. File a warranty claim for your Fitbit device - Google Help
The serial number is typically printed in very small text on the rear of the device. Fitbit Zip Serial Number
If the physical number is worn off or unreachable, you can find it in the Fitbit app by going to Account > Your Device, or via your desktop dashboard settings. Having this 12-digit/letter sequence saved ensures you can
By locating and storing your Fitbit Zip serial number, you can utilize it to verify ownership with Fitbit Support if your tracker is lost and subsequently found by another user. Fitbit Zip Serial Number