I have a feeling this could be right:

Subframes are prone to misalignment, which can cause vibration and alignment issues in the suspension and steering components. Misalignment is caused by space between the chassis-subframe mounting bolts and the mounting hole.

http://en.wikipedia.org/wiki/Subframe

Simple axle subframes usually carry the axle, the lower control arms and, in case of the front
axle, the steering rack. Subframes which also support the engine and possibly other
components (e.g. transmission) would be particularly useful on front wheel drive cars. Such
more complex, but also more expensive designs would result in better road isolation and less
harshness since these components are not anymore directly connected to the main body
structure. However, no specific examples can be shown.

http://www.alueurope.eu/wp-content/uploads/2011/11/AAM-Appli...



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Note added at 16 hrs (2013-06-29 14:27:58 GMT)
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http://www.mprovesolutions.com/page_2593205.html

When the automobile manufacturers began using subframe assembly methods in new cars, buyers were skeptical and sales dropped. It was thought, by the consumers, that the vehicles would not be as sound and strong as those possessing a full frame. Testing soon revealed however, that these vehicles with unit-body construction and a subframe were actually safer than the full-framed counterparts. Sales picked up and buyers felt comfortable driving and riding in the vehicles.

http://www.wisegeek.com/what-is-a-subframe.htm#didyouknowout

These are parts that bolt on to the chassis or frame - axle, steering components, suspension components, etc.
There are repair shops that specialize in underbody repair - Sears, Goodyear, Etc.

http://www.speedyalign.com/

http://rjrestorationsmn.com/restoration-services/underbody-s...