The movements grouped here are modern: nearly all of them formed within the last century, around a single founder, a new revelation or a distinctive practice. That is the loose half of the description. The part the grouping actually rests on is narrower: each of these movements came into conflict with the society around it, and the conflict left a public record.
Sociology of religion supplies the first half. What marks a sect or a new religious movement off from a settled church is tension with the surrounding world, and every movement here stood in such tension. What this atlas has added is a threshold. Collected under this heading are the cases where the tension stopped being a matter of unfamiliarity and became a matter of record: civil litigation, criminal conviction, official inquiry, and in a number of cases mass death.
That threshold is an editorial decision rather than a scholarly category, and it is set out here instead of being left to be inferred from the list. The selection criterion is notoriety. A reader is owed that plainly, because the alternative is a heading that looks like a neutral survey of modern religion and is not one.