A NCTC, FBI & BHS published document (32 pgs) and they explain at the start:
The indicators of violent extremist mobilization described herein are observable behaviors that could help determine whether individuals or groups are preparing to engage in violent extremist activities, such as conducting an attack or traveling overseas to join a foreign terrorist organization. Indicators are grouped by diagnosticity—meaning how clearly we assess the behavior demonstrates an individual’s likelihood of engaging in terrorist activity.
Shortly after a bit more:
... the initial list of indicators was developed based on a review of information derived from dozens of FBI terrorism investigations, peer-reviewed academic studies, and brainstorming sessions by experts from the Intelligence Community and law enforcement.

It is written in plain English and well laid out. There is an abundance of literature on the topic, sadly there are no pointers. There is a caveat about some activity is legal and constitutional, but when combined with other factors requires a review, if not action. It appears to have a use for the wider, general public.