Admission control guarantees vSphere HA failover by ensuring enough spare failover capacity within the cluster. If you have four ESXi hosts and all of them are utilizing almost 100% of the total compute capacity, and one of the ESXi hypervisors fails, the VMs will be restarted, but the remaining hosts won't be able to provide the required number of the resources other VMs thus performance will be degraded. Admission control takes care of that by reserving resources for the failover.
Configuration is at the cluster level under the vSphere Availability section, as follows:
You can define the host's failover capacity using one ...
