The "Global Entry number" that you provide to airlines is officially known as your Known Traveler Number (KTN) or your PASSID. This is a 9-digit number found on the back of your physical Global Entry card or within your TTP (Trusted Traveler Programs) account dashboard. When you book a flight, you should enter this number into the "Known Traveler Number" field in your passenger profile. Doing so ensures that the "TSA PreCheck" indicator is printed on your boarding pass, allowing you to use the expedited security lanes at U.S. airports. It is a common mistake to think the number is your "Global Entry ID"; however, for the purposes of airline reservations and secure flight data, it is always the PASSID. If you forget to add it during booking, you can usually add it at the check-in kiosk or by speaking with a gate agent, but it is much more efficient to save it directly into your frequent flyer profile so that it is automatically applied to all future reservations across different airlines.