Well put. A model requires both a skin and the model, which is a set of geometric figures that form a 3-dimensional figure. Without a skin, the model, in war3, would only be team color. When you have a skin, the majority have alpha channels, which let you either remove certain parts of a skin or let team color of the model wrap there. People can change the way this wrap looks by skinning it. But people who create models from scratch usually have to create new skins which are used to wrap around those. Lastly, some are geomerges, which use two different skins of units, and therefore, two skins, to wrap around different parts. Yes, a skin can use multiple parts which need individual skins. These are usually glows (sparkles), and occasionally, in some scratch models, covering different body parts
i.e. body, head, and lower body. Changing a model lets you change the geometric figures, as Form said, a footman compared to a demon hunter