Based on Marc's posts it sounds like NCRC likely has different definitions. These discussions around defining the use of cow's tails as prevention vs. protection seem irrelevant if we're all referring to the exact same use. There are obviously different styles of vertical caving in different caving regions, but I believe a good percentage of cavers are at times using their cow's tails clipped to something (anchor, traverse line, rebelay loop) with a little slack with the expectation that it can catch a small fall. If not, and the cow's tail is only used to hold body weight statically during rebelay crossings or when resting on an upper ascender, then it doesn't seem to matter much whether you use dynamic or static material, use knots or sewn terminations, or that you use anything more than 7mm cord. If there is some expectation that the cow's tail will catch a small fall without causing anchor failure, cow's tail failure, or bodily injury from the impact force, then it seems wise to reduce the potential fall factor, use knots instead of sewn terminations, and use dynamic rather than static tethers (perhaps in that priority order). It seems like some are arguing that if you do the first two then using dynamic rope is unnecessary, or provides an insignificant benefit. I'm not sure I agree, but without clear objective data or the proverbial trail of dead bodies I guess this is likely to remain a personal choice.
Not so much NCRC's view, but mine. If we got all the NCRC instructors in a room to discuss this I am sure that we would run out of oxygen long before opinions. Regarding the rest of your thread- I agree!
I think Scott is right on track with the slip vs fall. Cavers truly are never (in general SRT) in the position to take a true fall like rock/ice climbers are. This is why we do not use a dynamic component in our vertical systems. If you clip into a rebelay, edge line, or traverse line, you are preventing a fall- thus creating fall prevention. Losing your footing a foot or two is a "slip" not a true fall. Yikes, this can get verbally icky! This gets all real blurry when different rope user groups share data, reports, etc... since we tend to use some of the same equipment and terms but with different uses, meanings, or context. Cavers, climbers (rock and ice), mountaineers, arborists, industrial workers at height, sailors, mine rescue, S&M enthusiasts, etc... all use rope but can vary drastically beyond that.
To drag this out further I will throw out a question-
If I have a traditionally tied cowstail, or a Petzl prefab for that matter, and connect both long and short into an anchor- how many points of contact am I hanging on?