Often signifies a standardized measurement, such as a Rack Unit in server hardware.
If this is a Power Assembly, verify the total wattage/amperage capacity (usually found on the rear nameplate) before connecting high-load devices. U_M_P_A_4x18
Ensure your rack rails are compatible with a 4U height. If using an 18-inch depth, check that your cabinet has enough rear clearance for cabling. Often signifies a standardized measurement, such as a