Any notes containing other notes, i.e. in outline terms having child notes, is considered a 'container.' At the most trivial level this allows notes to be nested inside other notes. For instance, a 'Section' container might contain a series of 'Chapter' notes.
Containers are most useful for structured projects that will mostly be viewed in outline or chart view. In map view, only sibling notes within a a single container are visible on the map. Containers on the current map draw a 'viewport' showing a part of its child map.