What does RU (UR) label on DC power relays mean?
It is a UL label. From UL wikipedia article:


  The "Recognized Component Mark" is a type of quality mark issued by Underwriters Laboratories. It is placed on components which are intended to be part of a UL listed product, but which cannot bear the full UL logo themselves.[6] The general public does not ordinarily come across it, as it is borne on components which make up finished products.