Of course it starts with the military. A lot of people will tell you that the “RG” in “RG6” stands for “Radio Guide” which is all well and good except that there doesn’t seem to be any evidence of that. It’s like how some people think “SOS” stands for “Save our Ship” when it’s really just an easy way to send a distress call in Morse code.
What we can say is that the military spec known as the “Joint Electronics Type Designation System” defines cable type RG-6/U in some of its earliest specification lists, with type R meaning capable of being used for radio, type G meaning good for use with telegraphy (look this is an old standard, and “/U” applied to anything intended for universal use, in other words, it was good for civilians as well. The “6” part seems to be fairly random, as is the designation of other coaxial cables as RG5, RG58, RG59, and RG11.
Today there is a general agreement as to RG6 cable having a an 18AWG center conductor and 75 ohm impedance. There is no policing body stopping someone from calling an inferior cable RG6, although there are specifications which are more regulated.