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.
Agents are a special class of container, as their children consist of an an alias to every note matching the query stored in their $AgentQuery attribute.
Adornments (map view only) are not strictly containers but act like containers in that they have OnAdd and OnRemove actions and support the inside() action query.