Outlier Reports are a new feature that provides a way to surface misbehaving metrics within a homogeneous group of metrics, like CPU utilization on a DB-cluster, or request rates across a fleet of web servers, for example. These Outlier Reports are a new analytics visualization based on Metric Clusters. Each Outlier Report analyzes a cluster of metrics and shows a graph of standard behavior and outliers which don't match the group analysis.
Each Outlier Report shows you a normal range (in green) that represents the behavior of the "majority" of the metrics, and also a list of outlying metrics that were outside the normal range at some point. These metrics appear in the legend.