GuestHostnameGenerator

The virtual machine name specified by the client. This field is optional and it is only relevant when the value of HostnameGenerator.type is FIXED.

Base prefix, to which a unique number is appended. This field is optional and it is only relevant when the value of HostnameGenerator.type is PREFIX.

The HostnameGenerator.Type enumerated type specifies different types of Name Generators.
FIXED : Specifies that a fixed name should be used for the hostname for the virtual machine during the customization.
PREFIX : Specifies that a unique name should be generated by concatenating the base string with a number including the network name of the guest. Virtual machine names are unique across the set of hosts and virtual machines known to the VirtualCenter instance. VirtualCenter tracks the network names of virtual machines as well as hosts. VMware Tools runs in a guest operating system and reports information to VirtualCenter.
VIRTUAL_MACHINE : Specifies that the VirtualCenter should generate a virtual machine name. The name is generated from a base prefix comprising the virtual machine entity name. A number is appended, if necessary, to make it unique.

Virtual machine names are unique across the set of hosts and virtual machines known to the VirtualCenter instance. VirtualCenter tracks the network names of the virtual machines as well as hosts. VMware Tools runs in a guest operating system and reports information to VirtualCenter, including the network name of the guest.
USER_INPUT_REQUIRED : Indicates that the host name is not specified in advance. The user must provide a valid value when the actual customization spec is applied to a virtual machine.