If we are talking about server building. Mostly servers come in cases called chassis. They come in different heights starting with 1U-4U. Lets say you have a 1u server chassis with server hardware. Your limited on what you can fit in a 1U chassis compared to lets say a 4u chassis. a 4u Chassis can hold multiple of different kind of storage drives or even network cards, video cards, raid cards. The size of a chassis takes up space in a server rack or cabinet which is measured by RU that stands for "Rack Units". Another note is that all servers dont have to be in a server chassis. They can be in just a standalone computer case. It just comes down the functionality of the device it self that gives it that "Server" title.