Sounds easy enough, doesn't it? Just follow the rules, clearly stated, and inappropriate behavior is minimized, if not eliminated altogether.
I'm going to suggest not only is it not easy to determine, but that it is actually far more complex that many think, and that broad rules can only be a guide, not a guarantee.
First, the clearest rule-breaking inappropriateness, the one responsible for most getting into trouble on CC.....sharing personal info prior to a booking. This is easy to understand, providing revenue for CC, and keeping things formal until there is an approved meeting. It is inappropriate to send your e-mail, phone number, IG or FB, to a pro or a client, unless you are doing so after a booking proposal has been received and accepted. Then it makes sense.....we need to make sure that we have directions, a meeting place, etc for the upcoming appointment. But the action deemed inappropriate, the sharing of info, is fairly black and white. I was once flagged for doing so, as I did it prior to bookings being accepted, but without intent to harm. I was reprimanded, and I have followed the rules closely ever since. That was easy 
But the next issue, the one that crossed most of your minds when reading the discussion title, well, this one is far less clear. Nor should it be.
We hear all the time, especially in this forum, that this is a platonic site, as though that itself is clear. I started a thread about the complexities of that term recently. Thus, anything remotely sexual is meant to be forbidden, right? Completely inappropriate, yes? Well, that, my friends, is a tall order to determine! Are words and/or pictures that for some, conjure up images of sexual connection inappropriate? Or is there a gray area? Are photos clearly portraying cleavage, or scantily-clad scenes of frolic, inappropriate? Or a tongue licking lips? Or as the inexplicable fashion these days, tongue simply wagging from one's mouth?
Or how about innuendo in the wording of the profile? The kinds of openness that the reader can interpret to their imagination's intent?
My point is simply this: "Inappropriateness" encompasses a large area, much of which is gray. One could do a fairly simple study to determine which profiles receive the greatest number of inquiries, in the shortest amount of time, and compare the pictures posted to see if there might be a connection.
But moving on to the real substance of this issue......what happens in the context of a cuddling session. Between consenting adults. What is inappropriate, bearing in mind the "clearly" stated rules of the site.....what, exactly, is sexual behavior? Is it the touching of erotic areas on each other's bodies? Is it cuddling without clothes on? Is it kissing? If your answers to these questions were a resounding "YES!", let us explore further. What about kissing on the forehead? Or the cheek? Or a greeting kiss on the lips but with a closed mouth? Those seem to be non-sexual to me.....but to others, it may be too intimate, and arousing. But isn't it possible that the touching of non-erotic parts of the body can be arousing too? More for some folks than for others? And isn't it possible that the touching of even erotic parts of the body can be non-sexual, and non-arousing? And isn't it also possible that cuddling without clothes can be non-sexual for some? While any kind of cuddling, even fully clothed, can be sexual and arousing?
This is the crux of the matter.....it isn't easy to determine what's inappropriate from the outside. If two consenting adults have the matter in hand, and their personal boundaries are not violated, perhaps, just perhaps, it's not inappropriate.
I already hear the objections! Wait, CC is explicitly a non-sexual site! Even if two consenting adults agree to engage in sexual activity during a cuddling session, it is inappropriate! What say you to that?!?
Here's what I say......sexual energy is present in much of our sessions, even when there is limited, or no touching. Perhaps our asexual friends in the community will disavow such a suggestion, but many will understand what I am expressing. We cannot eliminate our imagination, our visceral responses, or the enlivening universal life force that sexuality is part of.....in our cuddling sessions. Nor should we want to. Sometimes that play, that gray area, where controlled sexual energy is, produces the most delicious goose bumps. Or electrified hair standing on end. Or pulsing energy, filled with love, traveling throughout our bodies. Sexual energy is part of all that......and is incredibly enlivening as well! Inspiring. Heart-pumping.
Maybe, just maybe, we can become comfortable with the paradox that is being human....an embodied spirit.....a being filled with confusion, empathy, love, and goodwill....intermixed with a hormone-filled animal body....and just try to figure it all out as we go along this incredible journey. We will make mistakes. We will learn about our bodies, and the boundaries of others. Men may learn how to slow down, enjoy the touch of the goddess, not self-shame about their eager visceral responses......while gifted women learn how distant men can become from their intentions once their bodies are aroused, and help them return to balance, with a knowing and loving presence.
All this is possible, in the context of the incredibly rich cuddling journey, even while we walk the tightrope of appropriateness, trusting our instincts and intuition, to guide us along this complex path. Rules may provide some security, but their application may differ as they're applied uniquely and artistically.
Grist for the mill, hey?