Every leaf page has an associated data page. The associated data page is where new rows are added. A leaf page can actually point to several different pages, but new data is only added to the associated data page. When an associated data page fills up, a new associated data page is attached to the leaf page. If you delete rows that exist on the current associated data page, the deleted space is reused.

Having one associated data page per leaf page provides a good chance for rows with similar key ranges to exist on the same data page, thereby increasing the likelihood that data references occur on the same data page.