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.

RU is an Underwriter's Laboratories mark for certified products that are intended to be used inside of other devices.

Reference: The UL site.

It is the Recognized Component Mark UL

As a basis for approval, code authorities look for UL Listing Marks to verify that products have been investigated for installation in the field in accordance with model codes. During inspections of electrical installations, code authorities may see a variety of certification marks, including the UL Listing Mark and UL Recognized Component Mark. To properly approve the installation, it is important for code authorities to understand the meaning of each of these marks as well as the important differences between them. The UL Listing Mark is the most common certification mark seen and accepted by code authorities.

http://ul.com/wp-content/uploads/2014/04/ul_RecognizedComponentMarks.pdf