The "Reel ID" as displayed by something like an EDL is the full clip name like "A001_C006_0427RU". The first portion, the "A001", being the camera and magazine/reel number. This value is created by the camera and, if converted in Redcine-X, will carry over into whatever other video format supports that metadata.
It really should be a 2nd assistant camera's operator's job to be sure the Real ID is correct. I believe it will automatically update, but from what I have heard, sometimes it will still update in error or if the counter hasn't been reset from a previous project, so it needs to be checked and manually updated to the correct number on the RED in those circumstances.