A hierarchy can be defined as 'an arrangement of items (objects, names, values, categories, etc.) in which the items are represented as being "above," "below," or "at the same level as" one another.' In the context of the Allegiance platform, a hierarchy is a way to organize data about your organization along various lines.
A hierarchy is essentially an axis along which you can manage, exploit, and enrich your survey data. It reflects some aspect of your organization - brand, region, organizational structure, etc. You can think of it as organizational metadata that you append to a survey in order to maximize the survey’s utility.