The spin theory of electrodynamics, dating back to the work of Sadovsky, Poynting and others, is called in the article the classical spin theory, in contrast to the currently widespread spin theory, which is a gradient theory. The uniqueness of the local densities of energy-momentum and spin of the electromagnetic field is demonstrated, and the energy-momentum is determined by the Maxwell tensor, which does not allow change. In particular, the Belinfante-Rosenfeld procedure is meaningless. The existence of spin as an internal angular momentum is confirmed experimentally. Mathematically, spin arises from the Principle of Least Action in the form of a spin tensor, along with energy, momentum and moment of momentum relative to some point. The use of the spin tensor made it possible to obtain new results concerning the emission and absorption of spin angular momentum. It is shown that the use of the spin tensor is the same natural process as the use of the energy-momentum tensor. The adequacy of the spin tensor provides the violation of the gauge equivalence of various vector potentials. In particular, it is demonstrated that the standard vector potential gives an incorrect value for the spin emission from a rotating dipole. Vector potentials related to each other by a gauge transformation are equivalent to each other only when calculating energy-momentum. The gradient theory of spin has been criticized. In particular, it separates the internal spin angular momentum from the energy and momentum of the radiation.