Yes, I know what you mean. Everything you add extra to your car has to be TUV approved and the shop will give you the paperwork to show the TUV testing garage, who will then issue you with your TUV certificate which is valid for 2 years.
It is a good system, as it means all parts are tested and proven safe, rather than here in the UK where people can stick any rubbish they want on their car which in extreme cases can make it a deathtrap.
As a rule of thumb, I think everything sold by a German tuner will have to be TUV approved. I have a German friend who builds race cars, if you want I can send you his email address and he can verify, as he knows the laws out there inside out.