I will try to answer with the most I have known.
Please feel free to help me if anything is lacking or incorrect.
The definition I noticed about Percentile is the value below which a percentage of data falls. In this case, the data set will be all samples collected from Dynatrace Agent from the source and along the time window you selected.
In your example, along the last 30 minutes, all key users actions were analyzed and sorted, and selected the very firsts samples above the 90%, to plot in the graphic, table, etc.
Hope I could help you.
See this interesting page describing the Percentile definition:
https://www.mathsisfun.com/data/percentiles.html
