Originally Posted by GUWonder

The US passport card does work on some international boat trips.

Airlines in Europe generally refuse to accept US passport cards to fly to the US even if the passport (book) is lost/stolen, so it generally cannot be used when coming from Europe. If a US citizen/national somehow arrives at the US port of entry without a US passport, then a passport card may be accepted as proof of identity and citizenship in much the same way as my regular old US state driving license could or even just reciting my passport number, SSN and name can.