 The Department of State stated that this limited validity is because "designing a card format passport for wide use, including by air travelers, would inadvertently undercut the broad based international effort to strengthen civil aviation security and travel document specifications to address the post 9/11 threat environment".Aside from being unable to be used for international air travel, the passport card is treated as a passport for all other purposes.A United States passport card can be used as primary evidence of United States citizenship, just like a passport booklet, and can be used as a valid proof of citizenship and proof of identity both inside and outside the United States.