Mdl Matrices

In mdl file, Vertex is controlled by bones, vertex -> vertex group -> corresponding bone groups -> bone,

Vertex = { Position, Normal , UV, vertex group, bones groups}

from vertex group to bone groups

Bone group, Matrices{ 0 }, 0 means bone with ObjectID = 1, which means this vertex is controlled by bone (with ObjectID = 1) matrix.

Sometimes, there are two or more elements in Matrices, for example Matrices{ 0, 1}

How does it works in this situation? Does a vertex is controlled by two or more bones? If yes, where is the weight?