  We consider the relativistic spinor field theory re-formulated in polar
variables so to allow for the interpretation given in terms of fluid variables.
After that the dynamics of spinor fields is converted as dynamics of a special
type of spin fluid, we demonstrate that such conversion into dynamical spin
fluid is not unique but it can be obtained through $19$ different
rearrangements, by explicitly showing the $19$ minimal systems of hydrodynamic
equations that are equivalent to the Dirac equations.
