Yes, type "U" refers to Unknown for the Nominal Annual Rate.

是的，“U”型指的是未知的名义增长率。

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For weeks, an unknown number of U.S. Special Forces troops have been involved in the effort that drove al Qaeda fighters out of the Tora Bora area.

CNN: Special Forces seen leaving Tora Bora

The bill's failure already has caused a steep drop in stocks and a plunge in prices of Treasury securities as investors flee the unknown for the seeming safety of U.S. government debt.

FORBES: The Banks

Amevive is the first treatment for psoriasis, a skin disease that affects some 4 million patients in the U.S. It occurs because patients' immune systems run amok for unknown reasons, causing painful lesions and rashes.

FORBES: Magazine Article