What is the meaning of U in the server?

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U is a unit that represents the external size of the server. It is an abbreviation of Unit. The detailed size is determined by the American Electronics Industry Association (EIA), which serves as an industry group. The reason for specifying the server size is to keep the server in proper size so that it can be placed on an iron or aluminum rack. There is a fixed server nut on the rack, which is aligned with the server's nut and fixed with screws.
The specified size is the server width (48.26 = 19 inch) and height (multiples of 445cm ). Because the width is 19 inch, the rack meeting this requirement is sometimes called "19 inch rack ".
The thickness is the basic unit of 445cm. 1u is 4.445, and 2u is 2 times of 1u is 8.89. That is to say, the "1u PC server" is a product that meets the EIA specifications and has a thickness of 4. 445cm. Products designed to be placed in 19 inch cabinets are generally called Rack servers.