The leaf page of an index is the lowest level of the index where all of the keys for the index appear in sorted order.
In clustered indexes on allpages-locked tables, the data rows are stored in order by the index keys, so by definition, the data level is the leaf level. There is no other level of the clustered index that contains one index row for each data row. Clustered indexes on allpages-locked tables are sparse indexes.
The level above the data contains one pointer for every data page, not data row.
In nonclustered indexes and clustered indexes on data-only-locked tables, the level just above the data is the leaf level: it contains a key-pointer pair for each data row. These indexes are dense. At the level above the data, they contain one index row for each data row.