U is unit of measure for vertical usable space, or height of racks, cabinets. 1U is equal to 1.75 inches (44.45mm).
Example : Form-factor of 3U cPCI board is 100mm x 160mm
Form-factor of 6U cPCI board is 100mm x 233.35mm
Note: 3U as per the definition is 3 x 44.45 mm = 133.35. But 160mm is defined in the cPCI specifications.