Single Scope Background Investigation

A Single Scope Background Investigation (SSBI) is a type of United States security clearance investigation required for Top Secret, SCI and Q access, and involves investigators or agents interviewing past employers, coworkers and other individuals associated with the subject of the SSBI. Standard elements include background checks of employment, education, organization affiliations and any local agency where the subject has lived, worked, traveled or attended school. These checks lead to interviews with persons who know the subject both personally and professionally. The investigation may include a NACLC of the subject’s spouse or cohabitant. Previous background investigations conducted on the subject may also be reviewed to corroborate the information obtained or disclosed within the new SSBI.

The Standard Form 86 (SF86) is required to begin the background check process. See the "Questionnaire for National Security Positions" at http://www.opm.gov/Forms/pdf_fill/sf86.pdf. SF86 is now an electronic form known as e-QIP, and the information on an investigation and its status is stored in either JPAS, PIPS or Scattered Castles.

...
Wikipedia