A rack-mount server is built to slide into a standard 19-inch equipment rack and is measured in units, each 1.75 inches tall. Security systems use them for video management, access control or storage. Racks need depth, ventilation, power and a UPS sized for everything installed.
The rack is the physical foundation of the system. A server that cannot be serviced or cooled will not stay online.
Fit, depth and airflow
Rails, depth and airflow direction matter more than the unit count. A deep server in a shallow closet blocks the door, and a full rack with no rear clearance recirculates hot air into its own intake.
Power and service access
Budget power for the whole rack, not one server, and leave room to pull a unit out on its rails. Label every cable, because the next person to replace a drive will not be the one who installed it.