Skin weights
The numbers that decide how much each bone influences a vertex — normalized to sum to one and usually capped at four bones per vertex for the GPU.
In skinning, every vertex gets a list of bones and a weight per bone. The weights are normalized so they add to one, otherwise the vertex would shrink or balloon. GPU pipelines almost always cap the list at four bones per vertex, since that fits neatly into a four-component vector.
The weights are set either by hand through weight painting — brushing influence onto the model in the DCC tool — or automatically at bind time via heat maps and distance heuristics. Sloppy weights show up immediately as surfaces that pinch or splay around a joint, and are the most common cause of ugly deformation long before you reach for a different skinning algorithm.