A 78-page document created by the National Counterterrorism Center setting forth the standards and procedures to be followed by intelligence and law enforcement agencies in placing individuals onto various terrorism-related watchlists, including the government's master watchlist and the No Fly List. The Guidance shows that inclusion on the master watchlist requires only "reasonable suspicion," a very low standard that can be met with uncorroborated or even questionably reliable information. The Guidance also permits watchlisting under numerous exceptions to the reasonable suspicion standard, including association with a watchlisted individual, even where that association may be wholly innocent.
Placement on a watchlist can have serious consequences, including an inability to travel by air or sea; repeated, invasive screening at airports; denial of permission to enter to the United States; or detention and questioning in the United States or abroad—to say nothing of shame, fear, uncertainty, and denigration as a terrorism suspect. See this report for more on watchlisting.