RG stands for military specs Radio Guide (which is old and meaningless anymore) and the RG numbers are kind of arbitrary. The most common RG designations seen these days are RG-6, RG-8, RG-11, RG-58 and RG-59. RG-58 and RG-8 are 50 ohm coaxes, unsuited for video work. RG-6, RG-59 and RG-11 are all 75 ohm cable types, with RG-59 being the smallest (20 AWG centre conductor), RG-6 in between (18 AWG), and RG-11 being the largest (14 AWG).