If you create a cluster with a default manual or partially automated mode, vCenter Server displays migration recommendations on the DRS Recommendations page.
The system supplies as many recommendations as necessary to enforce rules and balance the resources of the cluster. Each recommendation includes the virtual machine to be moved, current (source) host and destination host, and a reason for the recommendation. The reason can be one of the following:
Balance average CPU loads or reservations.
Balance average memory loads or reservations.
Satisfy resource pool reservations.
Satisfy an affinity rule.
Host is entering maintenance mode or standby mode.
If you are using the vSphere Distributed Power Management (DPM) feature, in addition to migration recommendations, DRS provides host power state recommendations.