A ceiling is a ceiling. It means that unlike a dive within NDLs, you no longer have the option of a standard ascent without incurring an increased risk of DCS. Just telling them to ascend slowly and the ceiling will probably go away is sort of like telling them just do the stop and the ceiling will go away. It's unplanned staged decompression by an untrained diver, and saying that's OK is normalization of deviance.
@doctormike, I think we are having a terminological difference only regarding the meaning of "ceiling."
I totally agree that we should not encourage recreational divers to casually incur
stops, hoping they will burn off during ascent. But, that's different than a ceiling. Where I disagree is that a ceiling "means you no longer have the option of a standard ascent...." I think that is the definition of a stop, or at least a slower-than-normal ascent in lieu of stop, not a ceiling.
A ceiling is simply the depth, at any given moment of the dive, at which your leading compartment would bump up against the limit of what is permitted in whatever model you are diving.
Many recreational dives will, perfectly within the "rules," generate a
ceiling without generating a
stop because a stop will only show up if the ascent rate used in the model is insufficient to clear the ceiling before you get there. We have ceilings all the time that clear during a normal ascent rate. A recreational diver would be oblivious to the existence of this theoretical ceiling unless it gets to the point that a normal ascent won't cure it. When a normal ascent won't clear it, then you have a stop.
For example, just playing with Subsurface at a 30/75 GF, a dive to 100' will generate a ceiling i
n only 10 minutes (the ceiling is 1 foot, but it's there). There's no deco
stop because -- at a standard ascent rate -- that ceiling clears almost immediately and has been gone for almost 2 minutes by the time you get to it.
I think that discussion is useful for recreational divers, assuming they are interested enough to follow it. It is common for them to disregard ascent rate warnings on their computers -- if they even know what they are. Understanding that they may indeed have a ceiling and they could bust it if they don't honor the 30'/min ascent rate is useful knowledge -- it is the explanation for why the ascent rate warnings actually mean something.
That's why I dislike the term NDL, because it incorrectly implies that decompression is something that only exists when you have stops and therefore as long as you don't bust the NDL you're free to shoot to the surface. It would be more accurate if we called it a "No Stop Limit".