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There is a passport app that lets you clear US customs much more quickly: (no pre-approval needed, only US/Canada citizens are eligible at present)

But you still need to carry the paper passport as backup.

just to note you can also use it as a non-US citizen if you are arriving on a Visa waiver programme (ESTA)

Additionally, Legal Permanent Residents of the US can use it too.

It says "Returning Visa Waiver Program Applicants". What does "returning" mean in this context?