A four-digit code you tell the driver at the car before the trip starts. By default it is the last four digits of your number, and it is asked on rides from 22:00 to 05:00. Under Account, Pickup PIN, you can choose Every ride, or choose your own PIN, which the app then does not show again. See The pickup PIN.
What is the pickup PIN?
Four digits you tell the driver at the car so the trip can start; asked at night by default.