The root level is the highest level of the index. There is only one root page. If an allpages-locked table is very small, so that the entire index fits on a single page, there are no intermediate or leaf levels, and the root page stores pointers to the data pages.
Data-only-locked tables always have a leaf level between the root page and the data pages.
For larger tables, the root page stores pointers to the intermediate level index pages or to leaf-level pages.