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OVF uses DMTF's common information model, or CIM standard. The main website for https://www.dmtf.org/standards/cim/ and the operating system information can be found hereat: https://schemas.dmtf.org/wbem/cim-html/2+/CIM_OperatingSystem.html. The DMTF issues new versions periodically, so check their website for updates. Also check which version your hypervisor provider is using.
Hypervisors have their own lists of supported operating systems based on specific versions of the CIM standard. For example, VMware has the VirtualMachineGuestOsIdentifier, which you can find in the VMware documentation.
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The details of the operating system are stored in the Abiquo template only, not in the VM definition. Therefore, if you change a template after you deploy a VM, the changes will not be made to the VM. If you do not supply an operating system version, then the platform will use the latest version of the operating system that you specified.
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