The
-model structure is responsible for various distinguished
features of the stationary Einstein-Maxwell (EM) system and
related self-gravitating matter models. This section is devoted
to a brief discussion of some applications: We argue that the
Mazur identity [133], the quadratic mass formulas [89] and the Israel-Wilson class of stationary black holes [102], [145] owe their existence to the
-model structure of the stationary field equations.