The “X” coordinate is an alpha character and the “Y” numeric, resulting in a unique identifier for each rack and cabinet. Like, pretend you and a friend are building out a 20U rack. From the bottom, there's a 2U UPS, a 4U NAS, and a 1U router. You want to tell them where to put the router, so you say "the router goes in 7". Each rack must be assigned to a site, and may optionally be assigned to a location within that site. Network racks, and the height of equipment that will be installed in them, are measured in ack units (abbreviated as Us). QR Codes: Store maintenance history, warranty info, or service contacts. What Is a Rack Unit (U / RU)? Ready to Optimize Your Server Rack Setup? Rack Units, or “U,” are the standard.