  A class of geometric structural fields of a given type is  characterized by a particular Lie group. A geometric structural field belonging to a given class has a microsymmetry group (see definition  4.1) at each point \(p \in M\) which is isomorphic to  the Lie group that is characteristic of the class. In relativity  theory, this microsymmetry group is isomorphic to the Lorentz group  and leaves invariant a pseudo-Riemannian metric of Lorentzian  signature. 