Which vertex buffer stream the attribute should be in.
Unity supports up to 4 vertex streams, most meshes use one stream for all attributes. Default value is 0.
For a mesh to be compatible with a SkinnedMeshRenderer, it must have multiple vertex streams: one for deformed data (positions, normals, tangents), one for static data (colors and texture coordinates), and one for skinning data (blend weights and blend indices).
Additional resources: Mesh.SetVertexBufferParams.