This category is the index of the atlas's own Group entities, the finer subdivisions filed beneath a Subcategory once an energy topic grows dense enough to need its own further split, such as a broad pioneer or technology subcategory sorted further by specific fuel type or era. A Group carries entries the same way a Subcategory does, one level further down the browse hierarchy; it exists purely as a structural device for organizing entries, not as an energy subject in its own right.