What Is a U on a Server Cabinet?

Hello all,



Generally, Cabinets enclose a rack, which is a frame that provides a means for mounting electronic equipment.

Cabinets come in a variety of styles, colors, and many contain baffles, fans, and other features. But, I am in doubt that what is a " U" on a server cabinet and its main purpose.


So, to clear my confusions I need some basic fundamental on it. I hope anyone having idea on it,

Please help.

Thanks!



I didn't find the right solution from the Internet.



References:
 www.cnet.com/forums/discussions/what-is-a-u-on-a-server-cabinet-575573/

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