The specification defines several critical dimensions to ensure a "form, fit, and function" match between cabinets and equipment: Rack Unit (U or RU)
This is the most famous metric in the standard. One Rack Unit is exactly 1.75 inches (44.45 mm) in height. A "42U" rack simply means it has 73.5 inches of vertical mounting space. eia-310-d specification pdf
: A minimum clear width of 17.72 inches (450 mm) to accommodate equipment. eia-310-d specification pdf