Falloff

The Falloff node generates the output color based on the angle between the face normal at the shade point and the view ray for linear and fresnel falloff type. While directional falloff type generates the output color based on the angle between the shade point normal and specified direction.

The linear and fresnel falloff types are different because the first one uses a linear falloff function, but the latter one uses fresnel dielectric function as falloff and accepts IOR value to change the function.