Shape keys are used to deform objects into new shapes for animation. In other terminology, shape keys may be called “morph targets” or “blend shapes”. Blender manual
Blender can extrapolate results when the Value goes lower than 0.0 or above 1.0. Blender Manual
github.com
A Morph Target is a morphable Mesh where primitives' attributes are obtained by adding the original attributes to a weighted sum of targets attributes. glTF 2.0 specification
unity3d.com
docs.unity3d.com
Sets the weight of a BlendShape for this Renderer. ... how much the Mesh has been blended (or morphed) from its original shape to a target BlendShape (another Mesh containing the same topology, but with different vertex positions than the original). Unity2020.1 スクリプトリファレンス
The BlendShape weight range includes values between the minimum and the maximum weights defined in the model. Unity 2020.1 documentation