The report defines "confirmed" as a preponderance of evidence, including fingerprints, DNA, photo match or reliable or well-corroborated intelligence that can identify a former detainee at Guantanamo.
The report categorizes as "confirmed" any former detainee identified as involved in terrorist activity by a preponderance of evidence -- such as fingerprints, DNA, photo match or reliable or well-corroborated intelligence.
My favorite example of this occurred a decade ago, when Boeing convinced the intelligence community that it could build the next generation of photo-reconnaissance spy satellites better than 40-year incumbent Lockheed Martin could.