Which aspect is NOT required when defining a hierarchy?

When defining a hierarchy, the primary aspects involve structural components that facilitate the organization and categorization of data. The reference to the table that the hierarchy is based on is essential as it indicates the dataset from which the hierarchy is derived, while the number of levels in the hierarchy is crucial because it defines the depth and granularity of the hierarchy structure. Additionally, the fields required for expressions are important as they dictate the specific data fields that will be utilized in calculations or expressions tied to the hierarchy levels.

In contrast, the importance of visualization is not a required aspect when defining a hierarchy. Visualization pertains more to how data is represented and perceived after the hierarchy is established, rather than being foundational to its creation. Thus, while effective visualization can enhance understanding and communication of the hierarchical data, it does not constitute a necessity during the actual definition process.