An important aspect of skiing safely is setting up the bindings and the one aspect of binding setup is the "DIN setting". The DIN setting affects how easily the binding releases and, while there are other binding setup aspects (forward pressure, height of the toe piece etc) this is one that everyone is mesmerized about.
One important aspect that we'll get out of the way first is that the DIN setting and the binding's functionality is influenced by the forward pressure - do not forget this important aspect. The forward pressure needs to be setup correctly for the binding to function as intended - it needs to be re-checked every time you touch the bindings and honestly, every time you click in to your bindings - get used to the sound they make when they're in a good known state and listen every time you click-in.
This is especially important if you share skis with others that may have re-adjusted the bindings, if you use older bindings etc.
There is a number of DIN calculators online, but the internets has no idea how this or that racer will ski, they will give you lawyer approved averages, which to be fair, are not far off, for the average skier. Even I walk out of my calculated Type 3 DIN, but in time settled on just slightly higher for most skiing, like an 9-10 on average.
The DIN calculated online is a starting point. For racers, use "type 3" and don't go lower than that.
You can start with the calculated DIN and think it through from here. If the skis come off too easily, in situations where they shouldn't, you can increase the DIN slightly. It is however customary to have all-mountain skis (if you're skiing the gnarly stuff) setup a little higher than on-piste skiing: there is more "twisting" and torque involved when skiing the gnarly stuff and there are situations off-piste where a binding releasing at the inopportune moment is worse than it not releasing.
However, for most, the calculated DIN is fine.
The settings are normally different for speed skis from Slalom skis for instance, so there's a first issue, with blanket averages.
As a coach, first you make sure the settings are appropriate roughly and send away the U16/19s in DIN 4 or 14, for instance. Anywhere in between is fair game at that age, each racer should have, remember and understand his or her own settings.
Second, you'll keep a keen eye for skis coming off during training or racing, in situations you don't think they should have come off, and recommend a binding check, sending a note that they seem to release too easy for the skier and let the parents talk the shop into setting higher (you cross out the questions and write the number :geek:). Or maybe not release when you think they should have. These days, most coaches are not allowed to touch bindings or make specific recommendations to parents, which sounds somewhat nonsensical, but it is what it is. How is a shop going to know how they ski in a GS course?
For your self or your own kids, you can also arrive at the right number as they grow and get better, like I did, by going up half a DIN every time they come off for no obvious technical reasons, until the skis stay on, at like 12.5 or 9.
As for skis coming off, the DIN is a just part of it. Poor pressure settings are a common culprit, even if the number looks ok. Ice build up at a race should get the coach in hot water but during training it happens more often, kids being kids. Improperly canted boots, dinged lips or chewed up soles etc - all play into this, at the lower ages usually.
Old bindings are an issue, diagonal release and plastic housing are things to avoid, in my experience.
Most commonly you get lower skilled racers skidding the wrong line in a course chewed up by higher level racers or a dad who found a screw driver and a binding manual. Been to those races and done that, sent them home in a stretcher with "tell your dad to never touch your bindings again if he likes you to spend more time at home than in hospitals".
In brief, body sizes and DIN calculators are a starting point. Don't go lower than that. And if you're asking this question, the 12 makes more sense ;) although I would be concerned with the housing quality and deltas for the lower bindings. It is common to find ski deals with stronger bindings - try to talk the shop into swapping it out for you, they might, or bypass the deal or buy bindings and keep the others for later, or your "bazaii" all mountains, which may have been sold to you with a 2-8 DIN but you want to get fast to the bottom :geek: we tend to crank the long all mountains as high as speed skis - jumping an iced up waterfall is no place for a plastic binding...