Service Tag is a unique identifier of a Dell server. You can use the service tag to view server specs and look up documentation, drivers & downloads for a specific product on Dell's Support website.
Locating the service tag of a server
The service tag of a server is displayed in the web interface of its iDRAC, which is Dell's out-of-band management platform. See «How to access the out-of-band management interface of a Dell server called iDRAC» for details.