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 also a measure of how much cluster imbalance across host (CPU and memory) loads is acceptable.
You can move the threshold slider to use one of five settings, ranging from Conservative to Aggressive. The five migration settings generate recommendations based on their assigned priority level. Each setting you move the slider to the right allows the inclusion of one more lower level of priority. The Conservative setting generates only priority-one recommendations (mandatory recommendations), the next level to the right generates priority-two recommendations and higher, and so on, down to the Aggressive level which generates priority-five recommendations and higher (that is, all recommendations.)
A priority level for each migration recommendation is computed using the load imbalance metric of the cluster. This metric is displayed as Current host load standard deviation in the cluster's Summary tab in the vSphere Web Client. A higher load imbalance leads to higher-priority migration recommendations. For more information about this metric and how a recommendation priority level is calculated, see the VMware Knowledge Base article "Calculating the priority level of a VMware DRS migration recommendation."
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.)