A customer wants to monitor 50th and 99th percentile response time in the Metric Browser...

Percentile metrics are a way to measure the distribution of response times for a Business Transaction. They show the value below which a certain percentage of measurements fall. For example, the 50th percentile metric shows the median response time, and the 99th percentile metric shows the worst-case response time for 99% of the transactions. To enable percentile metrics, you need to go to Configuration > Slow Transaction Thresholds and indicate 5 whole numbers between 1 and 99 as Percentiles to Collect. You can then view the percentile metrics in the Metric Browser under Business Transaction Performance > Business Transactions > > > Percentiles.

https://docs.appdynamics.com/display/PRO45X/Percentile+Metrics