Hierarchies
Hierarchies are quite special. A hierarchy is a group of multiple dimensions. However, a hierarchy doesn't just affect the data source, but also the visualization and the way the users can interact with it. A hierarchy creates a relationship between different dimensions, such as a parent-child relationship.
The dedicated icon for Hierarchies is 
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Note
Hierarchies are crucial for geographical roles. If you try to build a visualization in a city level without a hierarchy, lots of cities won't be displayed because their names are ambiguous (that is, they exist in multiple countries). By creating a hierarchy with a country-level field and a region-level field, there is no more ambiguity.
Dates, for example, are a hierarchy. A day is included in a week, which is included in a month and a year. If you use a date in a Worksheet, then Tableau automatically selects the YEAR and shows a small + in the pill, on the left-side of its name. This means that the dimension is a parent...
