Any notes containing other notes, i.e. having child notes in outline terms, 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 are visible on the map though a container on a map draws a 'viewport' showing a part of its child map.