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Re:US Senate Report on CIA Detention and Interrogation Program by andyjohnson0 on Tuesday December 09, 2014 @12:21PM (#8724058)
On the basis of googling "top secret noforn filetype:pdf", it seems that the convention is for the redacted text to be some kind of source classification (eg. "SI" for Signals Intelligence or "COMINT" for Communication Interception.

I have no idea what it would be in the case of this report, but my guess would be that it was redacted to avoid ambiguity given that "Unclassified" is stamped next to it, rather than because the classification is itself secret.

Re:US Senate Report on CIA Detention and Interrogation Program by dragonwriter on Tuesday December 09, 2014 @01:28PM (#8724614)
The classification is not secret, but the intelligence sources themselves likely remain classified, including the identification of the kind of sources supporting the information in the now-declassified executive summary.

Even vague descriptions of intelligence sources are often treated as more sensitive than the information derived from those sources.