Are you talking about resource pool shares or something else?
If it resource pools;
> Shares comes into picture, only when there is a contention of resources in the resource pool.
> Limit is something which makes sure that limit VMs are not allowed than that particular limit.
> Reservation is something which guarantees that the amount which is reserved for the VM will provided without fail.
--> If you set these values from VMs resource tab, it will be based on the availability on parent resource pool where the VM is actually located.
In cluster, I doubt there is a provision to provide resource reservations. The available resource reservation is nothing but fail-over capacity that the cluster tolerates.
Hope it helps.
~dGeorgey
