The Trending - Drilldown widget displays each level in order of highest to lowest rank. While ties are displayed with the same rank, peer levels are shown in gray and cannot be drilled.
How does it work?
The percent rank of a score is the percentage of other scores in its frequency distribution that are the same or lower than it. For example, a score that is greater than or equal to 75% of the other scores is said to be at the 75th percent rank.
Simply put, this is a method for comparing the performance of a particular level to that of other levels.
Remember to check a different help article detailing most of other settings of this widget.