The Domain Administrator needs to create a plant hierarchy because items in SmartPlant Instrumentation are organized at different hierarchy levels. For example, instrument tag numbers always belong to a plant hierarchy item at the lowest level (by default, this is a specific unit), whereas wiring panels belong to the plant hierarchy item at the highest level in the domain (by default, this is a specific plant). For the purpose of this Tutorial, you will be using the default plant hierarchy consisting of three levels — Plant, Area, and Unit.