 The number was assigned sequentially.The "RG" unit indicator is no longer part of the JETDS system (MIL-STD-196E) and cable sold today under the RG-6 label is unlikely to meet military specifications.In practice, the term "RG-6" is generally used to refer to coaxial cables with an 18AWG (1.024mm) center conductor and 75ohm characteristic impedance.