The complete 166-page "March 2013 Watchlisting Guidance" is here. and a related article is here . The document spells out the rules for putting people on the main terrorist database, as well as the no-fly list.

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...The heart of the document revolves around the rules for placing individuals on a watchlist. “All executive departments and agencies,” the document says, are responsible for collecting and sharing information on terrorist suspects with the National Counterterrorism Center. It sets a low standard—”reasonable suspicion“—for placing names on the watchlists, and offers a multitude of vague, confusing, or contradictory instructions for gauging it. In the chapter on “Minimum Substantive Derogatory Criteria”—even the title is hard to digest—the key sentence on reasonable suspicion offers little clarity:

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“To meet the REASONABLE SUSPICION standard, the NOMINATOR, based on the totality of the circumstances, must rely upon articulable intelligence or information which, taken together with rational inferences from those facts, reasonably warrants a determination that an individual is known or suspected to be or has been knowingly engaged in conduct constituting, in preparation for, in aid of, or related to TERRORISM and/or TERRORIST ACTIVITIES.”
The rulebook makes no effort to define an essential phrase in the passage—”articulable intelligence or information.” After stressing that hunches are not reasonable suspicion and that “there must be an objective factual basis” for labeling someone a terrorist, it goes on to state that no actual facts are required:

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“In determining whether a REASONABLE SUSPICION exists, due weight should be given to the specific reasonable inferences that a NOMINATOR is entitled to draw from the facts in light of his/her experience and not on unfounded suspicions or hunches. Although irrefutable evidence or concrete facts are not necessary, to be reasonable, suspicion should be as clear and as fully developed as circumstances permit.”
While the guidelines nominally prohibit nominations based on unreliable information, they explicitly regard “uncorroborated” Facebook or Twitter posts as sufficient grounds for putting an individual on one of the watchlists. “Single source information,” the guidelines state, “including but not limited to ‘walk-in,’ ‘write-in,’ or postings on social media sites, however, should not automatically be discounted … the NOMINATING AGENCY should evaluate the credibility of the source, as well as the nature and specificity of the information, and nominate even if that source is uncorroborated.”

There are a number of loopholes for putting people onto the watchlists even if reasonable suspicion cannot be met.

One is clearly defined: The immediate family of suspected terrorists—their spouses, children, parents, or siblings—may be watchlisted without any suspicion that they themselves are engaged in terrorist activity. But another loophole is quite broad—”associates” who have a defined relationship with a suspected terrorist, but whose involvement in terrorist activity is not known. A third loophole is broader still—individuals with “a possible nexus” to terrorism, but for whom there is not enough “derogatory information” to meet the reasonable suspicion standard....
Onward and upward,
airforce