I have learned that there are Models and non-Models. Two very different kinds of people.
In order to be called 'Models' they have either been through some kind of schooling in 'modelling' or have learned 'modelling' through practice—doing it a lot with photographers who want to do 'model shoots'. Some models are professionals or semi-professionals, in the sense that they earn money doing this; possibly a lot, but mostly rather little, if for no other reason but that there are so many of them.
As models develop their craft—for a 'craft' it is—they develop habits. Every craftsperson does. In the modelling domain, those habits have to do with poses. How you hold your torso and legs, your head, your facial expression, your gaze, your hand; what you do with your hair. Standing, sitting, lying down.
If you study model images carefully—just google "model photography" and look at the images that come up—you'll soon find that there a finite, not very large, number of basic poses, each of which has a number of variants. If you wanted to, you could make up a whole catalog of images, and label them something like 'Pose 23, Variant 15'—or maybe '23.15' to keep it short.
Some poses have more variants than others, and of course every model will add their own characteristics to, say, pose 19.5, which then becomes X19.5, with X being the name of the model. I know this sounds rather cold, and it is. I'll discuss this further down, so please bear with me.
The advantage of working with Models is that they have the skills and the pose-repertoire, and as a photographer you can often go on autopilot and just shoot away happily while your model(s) go through their collection of poses. Just rattle off the pose-code and Bob's your uncle. Each element of this collection has what almost amounts to an 'industry standard' meaning; stuff it's meant to express. Look at a magazine with lots of photos of models, male or female, and there's a tacky kind of predictability about it all.
They have not been through a training process and usually don't know about any of that stuff. Means you may end up having to tell them literally everything. They're awkward, have none of the industry savvy or skill set, and their state of mind is often best described as 'confused'.
I hasten to say that there's nothing wrong with that! Nothing at all. But, unless they're 'naturals', non-Models are, at least with regards to poses, a bit like blank canvases. More work as a result and also more of a challenge to the photographer. (The same, by the way, applies when a Model is suddenly confronted with a non-standard pose. If they're skilled and mentally flexible they'll pick it up soon enough; but those who aren't, may have some issues and be prone to falling back on poses they're familiar with. That, too, can be a challenge to the photographer.)
Working with Models and non-Models