Tag used for questions pertaining to hardware being certified for various uses, whilst keeping within the scope of Ubuntu related questions.
Tag used for all questions pertaining to official certification by Canonical as outlined on the following sites:
The first explains how to obtain certification and defines what "certified hardware" represents, as well as explaining the key features.
The second is a relatively complete list of compliant hardware with the Ubuntu operating system be it server or PC.
Note: The list of certified hardware isn't to be taken as a list of "The only working pieces of hardware" as not every piece of hardware working with Ubuntu will be certified, especially when the hardware has been tested by individuals and not manufacturers.