Components are required for group types
Categories and groups can have components. Components of group and category types display in the group and category views. Item types do not have components.
Group types represent actual data objects, so groups must have components. The one exception is a partitioned group which is explained in "Partitioning".
By contrast, a category is used for organizing types and for type property inheritance reasons. Categories do not define actual data objects in detail. A category does not need components.
Each group must have at least one component, unless it is partitioned.