Sensitive Compartmented Information (SCI) is a classification label that is put on data and information that is sensitive in nature and belongs to a certain program or department. The data can be derived from multiple sources as Critical Program Information (CPI), analysis data and/or intelligence data. The SCI moniker is normally used by the National Intelligence Agencies and can be used for data that is classified as confidential all the way up to Top Secret (TS). Access to this data is only allowed for people who have the appropriate SCI ticket with their clearance.
Eligibility for access to SCI is determined by a Single Scope Background Investigation (SSBI) or periodic reinvestigation. Because the same investigation is used to grant TS clearances, the two are often written together as TS/SCI. Eligibility alone does not confer access to any specific SCI material – it is simply a qualification. One must receive explicit permission to access an SCI control system or compartment. This process may include a polygraph or other approved investigative or adjudicative action. Once it is determined a person should have access to an SCI compartment, they sign a Nondisclosure Agreement (NDA), are “read on” or indoctrinated, and the fact of this access is recorded in a local access register or in a computer database. Upon termination from a particular compartment, the employee again signs the nondisclosure agreement. 
SCI control systems may be the most well-known intelligence Special Access Program (SAP).
- The intelligence community itself considers SCI and SAPs distinct kinds of controlled access program
AcqLinks and References:
-  Website: Wikipedia – Sensitive Compartmented Information