Back of a Black & Decker Air Station. Can be powered from a standard plug (AC) or a car lighter (DC), and you choose which. The design doesn’t assume you know which is AC and which is DC.

Actually, it gives various opportunities to figure out which is which, and when all else fails provides the icon of a car to make it absolutely clear. It’s ensuring understanding and educating at the same time.

Labels are good. Labels that assume the user has a specialized vocabulary are not. For example a button that says ‘Check If This Username is Available’ is much better than a button that says 'Validate The Uniqueness of this ID’.

Be sure your labels do not make unreasonable vocabulary assumptions. Better, teach the user new vocabulary through the interface design.