The first texture applied to a slice is the data texture. It is a 3D texture with three values per sample point: one for primary data value V, one for the gradient magnitude G, and one for the second derivative. Since we measure this quantity using a Hessian, we call it H.
Next is the transfer function. Since we are dealing with 3D transfer functions, it is represented either as a 3D texture where each axis maps a variable from the data texture, or it can also be composed as a 2D and a 1D texture.
Normals for shading are stored in a separate texture. They are derived from the normalized gradient vectors at each sample. The normal texture is a 3D texture with three values at each sample point.