Parent sub-system and Root sub-system
In an hierarchy of sub-systems, the parent sub-system of a block or sub-system is the sub-system immediatly above it in the hierarchy.
For instance, if the sub-system Forest unit contains sub-systems named Forest, Berries, Mushrooms and Game, then Forest unit is the parent sub-system of Berries sub-system.
The top-level sub-system of a model. The id of the root sub-system is blank, so that blocks that belong to it have the same id as name.