What is an HA slot?
a slot is a given to each host in an HA cluster, there is a maximum of 32 in an HA cluster
a slot is given to each virtual machine, this determines the restart priority
a slot has nothing to do with HA, its how DRS calculates which virtual machine should be placed where
a slot is a logical representation of the memory and CPU resources that satisfy the requirements for any powered-on virtual machine in the cluster