There is only one file type that uses the .VMSS extension:
VMware Suspended State File
File created by VMware virtualization software; saves the state of the virtual machine (VM) when it is in a suspended state, meaning that it has been stopped temporarily; used for resuming the VM from where it was stopped; created automatically by the software when the VM is suspended.
The suspended state allows VMs to be stopped and deallocated from using system resources. This is helpful because users that have multiple VMs can switch between them without overloading the system.
NOTE: The suspended state differs from a snapshot (.VMSN and .VMSD files), which is an explicit save of the VM at a particular point in time.
Updated: March 27, 2012