What is the difference between enabling it or not, please find the details below if you enable it:
- Memory reserved to a VM is guaranteed never to swap or balloon.
- If an ESXi host does not have enough unreserved RAM to support a VM with a reservation, the VM does not power on.
- Adding a vSphere DirectPath I/O device to a VM sets memory reservation to the memory size of the VM. Typically this is required by pass-thru-related features such as DirectPath I/O and SR-IOV that allow the VM to access PCI devices directly bypassing the virtualization layer.
Selecting this check box ensures that all of the VM's memory gets reserved even if you change the total amount of memory for the VM. The memory reservation is immediately readjusted when the VM's memory configuration changes.
- VMs never consume more physical RAM than is specified by the memory allocation limit.
- VMs might use the VM swap mechanism (.vswp) if the guest OS attempts to consume more RAM than is specified by the limit.
For more information please refer to this 3rd-party
article.