The reason for specifying the server size is to keep the server in an appropriate size to be placed on an iron or aluminum rack. There is a screw hole on the rack for fixing the server. Align the screw holes of the server and fix them with screws.
The specified dimensions are the width (48.26 cm = 19 inch) and height (multiple of 4.445 cm) of the server. Because the width is 19 inch, the rack that meets this requirement is sometimes called "19 inch rack".
The basic unit of thickness is 4.445cm. 1U is 4.445cm, and 2U is 8.89cm, which is twice as much as 1U. That is to say, the so-called "1U PC server" is a product whose appearance conforms to EIA specifications and its thickness is 4.445cm. Products designed to fit into 19 inch cabinets are generally called rack servers.