Backed texture : limit?

Hmm
Reading this post it is not per vertex but a matrix per bone.

Quick question before I deal with it tomorrow: is each matrix local to the bone? So the value has to be considered relative to the root of the mesh and then, one can apply the translation as needed?
3x3 or 4x4 matrices?
The base of those matrices is the gltf’s bind pose of each bone?