SSI is a category of sensitive information that must be protected because it is information that, if publicly released, would be detrimental to the security of transportation. In other words, SSI is information that could be used by our adversaries to bypass or defeat transportation security measures. For detailed categories of SSI, see the SSI Regulation, 49 C.F.R. § 1520.5(b)(1) - (16).
Note: Under 49 C.F.R. § 1520.5(a), the SSI Regulation also provides other reasons for protecting information as SSI. TSA, however, primarily uses the criterion of “detrimental to the security of transportation” when determining whether information is SSI.