"U" is height unit for outside size of server, the full world is "Unit", the detail size standard is regulated by EIA.
1U=44.45mm=1.75 inch
1 inch=25.4mm
19" =482.6mm
On the mounting profile, has screws holes to fix the servers screws holes, then use screw and nuts to fix the severs on the mounting profile.
And the width of the sever should be 19" (19" =482.6mm) and the height should be "U" (or times of "U"),because the severs are 19 inch, then sometimes we say the cabinet which can meet 19�� use to be "19 inch cabinet".

19 inch standard cabinet:
19 inch is the width, can put the 19" severs and other 19" equipments.
Cabinet has U and depth, usually, the height is 42U,and here 1U and 19 inch, both are regulated by EIA.
The standard cabinet structure is simple, which including:
Basic frame, inside support system, cabling system, ventilation system.
19 inch cabinet has width, depth and height. Although the 19 inch mounting width between each mounting ear holes is 465.1mm, but usually the actual width for cabinet has 600mm and 800mm width. And the height is from 0.7M-2.4M(based on inside equipments qty and specification), the normal height is 1.6M and 2M. And the depth is from 400mm-1000mm,based on inside requirement. And factory also can make any depth cabinet, the normal depth is :450mm,600mm,800mm, 900mm,1000mm.

And the equipments need to be put in the cabinet, the height unit is U,1U=44.45MM.And the mounting panel width usually is 19 inch, and if the equipment is not 19 inch, then will make some 19 inch panel to fix the equipments and then put in cabinet.
42U height, but 42U cabinet can not put 42pcs 1U servers, usually only put 10-20pcs servers, because among the servers need ventilation and rejection of heat.