Categories (and subcategories) are used for classifying resources inside of a space.
An administrator can define them, can be assigned to when creating resources, and then participants can filter the different resources by these categories. For instance, they can be set up in proposals, meetings and debates.
As an example of this mechanism in action you can see it in Propose new functionalities for Decidim software process:
<aside> ℹ️ Once a category is in use it cannot be removed, as there are resources that are using it.
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<aside> ℹ️ Categories can be nested, meaning that a category can have subcategories.
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Decidim has other ways to classify resources, for instance Scopes and Hashtags. The main difference is that Categories can be defined in a given space, Scopes are global for all the platform, and Hashtags are defined by the participants.