[Element name] Lun [Description] 
Optional. Specifies the Logical Unit Number (LUN) for the data disk. If the disk is the first disk that is added, this element is optional and the default value of 0 is used. If more than one disk is being added, this element is required. You can use Get Role to find the LUN numbers that are already being used. Valid LUN values are 0 through 31.