Animation layer
Several animations stacked so that each affects a subset of the skeleton — a base locomotion loop for the legs, an upper-body layer that aims — blended with weights and masks.
A character often has to run and shoot at once. Rather than author a bespoke "run-and-shoot" clip for every combination, you split the skeleton into layers: a base layer plays the run cycle while an upper layer, with a bone mask, lays the aiming pose on top. Each layer has a weight and a blend mode — override or additive (see additive animation).
The mask decides which bones the layer touches; the rest are left to the layers beneath. Unity's Animator and Unreal's anim layers are built on the pattern. Each layer is usually fed by its own state machine and blended into the final pose before the skeleton deforms the mesh.