The easy way to remember is: U = Useable height. and that 1U is equal to 44.5mm.
For example, a rack-mounted piece of equipment that is described as 1U will consume 44.5mm of vertical rack mounting posts within the rack enclosure.
Equipment measured in U height is intended for horizontal mounting.
"U" should not be used to measure the external dimensions of a cabinet.
This all applies to both commercial 19-inch racks and SoHo 10-inch rack equipment and enclosures.