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. From 2003 to 2006 the Federal Bureau of Investigation issued 192,499 national security letter requests.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.