When Discrete ordinates method is selected, the components of each discrete ordinate vector can be used in this expression. The syntax is name.sx, name.sy, and name.sz, where name is the physics interface node name. By default, the Heat Transfer interface is ht so ht.sx, ht.sy, and ht.sz correspond to the components of discrete ordinate vectors.

When P1 approximation is selected as the Radiation discretization method for the physics interface, there is additional theory, equations, and variables described in P1 Approximation Theory.