With tag hierarchies, you can create subtag categories that connect to a top tag category.
It will allow you to limit the options to choose from in a tag and subtag category group.
If for example, your organization has a structure where a set of "Regions" belong to a specific "Market", you could create a top tag category called "Market" and then add "Region" as a subcategory.
In order to create a Tag Hierarchy go to settings and then Tags, choose the desired tag category.
At the righthand side of the main tag click at then enter a name and an optional description. To add Sub-tag options click at next to each top tag category.
Calendar tags cannot be added to a hierarchy.