$Container holds the '/' terminated path to the current note. The attribute can be set by the user and in doing so the note is moved to a new parent container. Such an approach is useful if processing notes that then can be archived away from still-current content.
Using $Container with agent actions. Care needs to be taken as agents act on aliases. Therefor an agent action seeking to move a note must act on the original of the note, i.e. via $Container(original).