Jay, I'm not going to touch BTZS use of terminology, but Subject Luminance Range does only refer to the scene. The luminance from the scene is then illuminant upon the film after it enters the camera. At which point it is referred to as the exposure E, the amount of light falling on the film in one second, or the photographic exposure H with represents the exact amount of light striking the film in a given exposure. Remember the old I x t = E? It's now E x t = H. Exposure times time equals photographic exposure. This equation is good for either sensitometric exposure or camera exposure. Of course, finding E is a whole other story.
My concern about Davis' use of the different terms is that while it works within the limited number of people using BTZS, it creates confusion when attempting to communicate the ideas to the outside community. My personal opinion why Davis uses such outdated terms such as SBR and ES is because they were used in the seminal papers Davis must have read when learning theory. He used them in the first edition and then was stuck, so in the later editions he was forced to use a disclaimer. Personally, I think he should have corrected the terminology before it became too engrained. Note: The BTZS part is just conjecture and opinion, so please do not consider it a call to arms.