in my point of view, { Dense, Sparse } and { key, non-key } both are different sets
we use
1) key + file is ordered by key ===> uses Sparse Index ===> Primary Index
2) non-key+ file is ordered by non-key ===> uses Sparse Index ===> Clustered Index
3) non-key+ file is not ordered by non-key ===> uses Dense Index ===> Non-Clustered Index
4) key+ file is not ordered by key ===> uses Dense Index ===> Secondary Index
ordering of data record is same as or close to the ordering of data block in some index it can be primary index or clustered index