The system hierarchy is the default hierarchy that MicroStrategy sets up for you each time you create a project. It contains all of the attributes in the project and is actually part of the schema definition. When you first create a project, the only hierarchy it contains is the system hierarchy.
The system hierarchy holds information on the relationships between attributes in the project. The system hierarchy cannot be edited but is updated every time you add or remove attribute children or parents in the Attribute Editor, or when you define attribute children in the Project Creation Assistant.
The system hierarchy is useful in determining relationships between all objects in the project. Attributes from the system hierarchy do not need to be part of an explicitly-defined user hierarchy. Any attributes that are not assigned to a user hierarchy remain available to the system as report objects, filter conditions, and components of consolidations. These report objects are discussed in detail in the Advanced Reporting Guide.
You can view the system hierarchy in the Data Explorer or in the Hierarchy Viewer, but not the Hierarchy Editor. You can access the Hierarchy Viewer from Graphical View in the Schema menu. The Hierarchy Viewer is discussed in detail in Using the Hierarchy Viewer.