What UR / RU symbol found on comsumer electronics mean

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. The general public does not ordinarily come across it, as it is borne on components which make up finished products.

These marks are specifically used on component parts that are part of a larger product or system. These components may have restrictions on their performance or may be incomplete in construction. The component recognition marking is found on a wide range of products, including some switches, power supplies, printed wiring boards, some kinds of industrial control equipment and thousands of other products.

In short it simply means the particular item, in this case the PCB, is not a 'standalone item' it is intended to be used within a larger system.