What is the difference between Data Categories and Category Groups in Knowledge?

Category Groups are the top-level containers that organise Data Categories — for example, "Products" or "Geography." Each Category Group defines a dimension of classification. Data Categories are the individual classification values within a group — for example, under "Products": "Cloud Storage," "CRM," "Analytics." Articles can be assigned to one or more categories. Data Category Visibility (set via profiles or permission sets) controls which categories — and therefore which articles — users can see. Category Groups can also be used to control article visibility in Experience Cloud communities.