how to calculation cosine between a vector and a plane ?

[tiantianxuabc]

I can't understand why this ?

const Vec3_t epiplane_in_2 = E_21 * bearing_1;
const float cos_in_2 = epiplane_in_2.cross(bearing_2).norm() / epiplane_in_2.norm();

[tiantianxuabc]

vector: epiplane_in_2 and bearing_2
plane params(a,b,c) or plane normal: epiplane_in_2.cross(bearing_2)
cos = ||vector|| / ||vector|| ????