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 of the following options:

• Disabled: No admission control is configured, and no resources are reserved for failover.
• Slot Policy (powered-on VMs): A slot is a logical representation of memory and CPU resources. A slot is the memory and CPU reservation required for any powered-on VMs in the cluster. Slot Policy can do good work in the environment where there is a very similar VM with the same CPU memory configuration. When you have a lot of small VMs and two monster VMs, it is not a very good situation because the reservation selects the most significant value. You can change this value for the CPU slot size and memory slot size through Advanced Options.
• Cluster resource Percentage: You can design vSphere HA to perform affirmation control by holding a particular level of group CPU and memory assets for recuperation from host failure. vSphere HA calculates CPU and memory. CPU calculation uses CPU reservation for powered-on VMs. If you don't use reservation HA, use the default value of 32 MHz. The memory calculates the memory reservation and memory overhead of each powered-on VM; the default value is 0 MB. You can override the calculated failover capacity.
• Dedicated failover hosts: This is the last option for defined failover hosts. You can specify and dedicate failover hosts. vSphere HA uses such hosts when it needs failover actions or has insufficient resources.

You can find more information about different Admission Control policies at https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.avail.doc/GUID-85D9737E-769C-40B6-AB73-F58DA1A451F0.html.

Then, there is a new option called Performance degradation VMs tolerate. This new setting in vSphere 6.5, if set, will issue a notice when a host disappointment would cause a decrease in VM execution depending on the genuine asset, not simply arranged reservations.