RG6 and RG6/U are types of coaxial cable. They are most commonly used to route cable television signals. RG stands for "Radio Guide," derived from the original military specifications for coaxial cables used to carry radio waves for communication. Electrically, RG6 and RG6/U cables are identical. Both have a size 18 American Wire Guide (AWG) centre conductor and 75 ohm impedance.
There is no precise designation for the meaning of the letter U in RG6/U coaxial cable. One theory attributes the meaning of the letter U to the word underground.
Another theory regarding RG6/U cable is that the U signifies resistance to ultraviolet light or UV resistance. Because of this, and the possibility that the U signifies underground, it is recommended you use RG-6/U cable instead of RG6 cables outdoors.
A third theory regarding the U in RG6/U is that it derives from the word "universal." As a universal cable, it can be used for any purpose befitting an RG6 cable. This would also imply that there is no difference between RG6 and RG6/U cables.