The DRS migration threshold allows you to specify which recommendations are generated and then applied (when the virtual machines involved in the recommendation are in fully automated mode) or shown (if in manual mode). This threshold is a measure of how aggressive DRS is in recommending migrations to improve VM happiness.
You can move the threshold slider to use one of five settings, ranging from Conservative to Aggressive. The higher the agressiveness setting, the more frequently DRS might recommend migrations to improve VM happiness. The Conservative setting generates only priority-one recommendations (mandatory recommendations).
After a recommendation receives a priority level, this level is compared to the migration threshold you set. If the priority level is less than or equal to the threshold setting, the recommendation is either applied (if the relevant virtual machines are in fully automated mode) or displayed to the user for confirmation (if in manual or partially automated mode.)
Each migration recommendation is computed using the VM happiness metric which measures execution efficiency. This metric is displayed as DRS Score in the cluster's Summary tab in the vSphere Client. DRS load balancing recommendations attempt to improve the DRS score of a VM. The Cluster DRS score is a weighted average of the VM DRS Scores of all the powered on VMs in the cluster. The Cluster DRS Score is shown in the gauge component. The color of the filled in section changes depending on the value to match the corresponding bar in the VM DRS Score histogram. The bars in the histogram show the percentage of VMs that have a DRS Score in that range. You can view the list with server-side sorting and filtering by selecting the Monitor tab of the cluster and selecting vSphere DRS, which shows a list of the VMs in the cluster sorted by their DRS score in ascending order.