NSL definition from Wikipedia (yes syncretic Wikipedia)
Notice the definition of non-content information. That's a relief. It's the content that can be really damning.
A national security letter (NSL) is a demand letter, which differs from a subpoena. It can be used by US government agencies, mainly the FBI, when investigating matters related to national security. It is issued to a particular entity or organization to turn over various record and data pertaining to individuals. NSLs can only request non-content information, such as transactional records, phone numbers dialed or email addresses mailed to and from. They also contain a gag order, preventing the recipient of the letter from disclosing that the letter was ever issued.
From 2003 to 2006 the Federal Bureau of Investigation issued 192,499 national security letter requests.