When you clone a virtual machine or deploy a virtual machine from a template, you can customize the guest operating system of the virtual machine to change properties such as the computer name, network settings, and license settings.
Customizing guest operating systems can help prevent conflicts that can result if virtual machines with identical settings are deployed, such as conflicts because of duplicate computer names.
In the vSphere Web Client, you must select an existing customization specification. To create customization specifications, you must use the vSphere Client. See vSphere Client online Help.