Which OS are you trying to create a pool for? Custom specifications are the "answer key" (if you will) for SysPrep. These are created on the vCenter server and you'll need appropriate permissions (if you are not a vCenter administrator) to make these. You need one for each unique installation Guest OS (Windows or certain linuxes) (e.g., if you had 3 different installs of XP that had different requirements you'd have 3 custom specs; if however you used a volume license and they all had the same basic settings with DHCP and used the VM name for the computer name, you'd need only one).
You only need to "install" the sysprep files when provisioning XP. Vista and higher have the SysPrep files natively installed.Image may be NSFW.
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