RG refers to old [Mil-Spec] cable identifications such as RG-8/U. "RG" means radio guide. These ratings are only used as reference today since RG is no longer used in the Mil-Specs. RG-8A/U is now M17/163-00001.

An RG Type cable is one that is manufactured similar to, but not exactly the same as, the [Mil-Spec] specifications.

!!! RG-U Defined
RG means Radio Guide. U means Universal.