I recently had to renew my passport and I decided to get a passport card in addition to the passport book. They just came yesterday and with the passport card came a letter telling me to check the passport card for correctness and to be careful with it and suggesting I keep it in a sleeve they included. At the bottom of the letter in large bold type it says "The U. S. passport card is not valid for air travel."

Does this mean that I cannot use it for identification when I go through security on on a flight? I am surprised by this since the TSA's website says that "Acceptable IDs include: ... the U.S. passport card". So I will assume that it is acceptable for flights but if that is the case, What is the State Department saying in the letter they sent with the passport card saying it is not valid for air travel? Any know what is going on? Maybe on my next flight I will bring the passport card in addition to my drivers license and see if I can get through security with the passport card. I can't image why it wouldn't be acceptable but I don't understand what the the letter was trying to say.