I took the reference from other blog - slmgr tool is "Software Licence Manager" :"SLMgr Commands and Options
I’ve been dealing a lot lately with activation of Server 2008. It’s not been as easy as I’d like. In searching for some help, I came across a great article on SLMgr: SLMgr Commands and Options with Windows Vista Product Key Activation. This article provides a good look at the options with this SLMgr.ext tool. "