The RG6/U specification dates back to WWII if I remember correctly. "RG" stands for "Radio Guide" and was a military publication outlining specifications for cables.
In terms of modern cable, it means nothing more than a coaxial cable of 75 ohms impedance with an 18 gauge center conductor.
It does not specify the shielding or the type of dielectric (foam, solid plastic, etc.)
Typically cable companies recommend (at a minimum) double-shield cable with a 100% aluminum foil shield and a 60% aluminum braid and a foam dielectric. The foil shield should be "bonded" to the dielectric..that is, glued to the dielectric so it does not get pushed out of the way as you install the connector.
There is also "tri-shield" cable which consists of a 100% AL foil, a braid of usually 60% AL, and another 100% AL foil.
Then there is "quad-shield" cable which consists of a 100% AL foil, a 40% AL braid, a 100% AL foil, and a 60% AL braid. Comcast and Cox both use this cable for customer wiring. (Commscope F6SSVV is the brand and model of the cable they use).
All of these are usually specified with a "copper covered steel" center conductor for cable TV use, although they are available with a full copper center conductor.
Full-copper center conductor is recommended for CCTV and power-passing applications due the lower resistance of the center conductor.
For cable TV applications a copper-covered steel conductor is probably better since it's less likely to get bent up as the connector is plugged in.
Finally, we have single-shield cable which consists of a braid and no foil (typically this has a copper braid). This is OK for CCTV applications. NOT recommended for cable TV applications.
