  Considering the importance of the flexoelectric coupling for the physical
understanding of the gradient-driven couplings in mesoscale and nanoscale
solids, one has to determine its full symmetry and numerical values. The
totality of available experimental and theoretical information about the
flexocoupling tensor symmetry (specifically the amount of measurable
independent components) and numerical values is contradictory indicating on a
limited understanding of the coupling properties. However the discrepancy
between the theory and experiment can be eliminated by consideration all
possible inner symmetries of the flexocoupling tensor and physical limits on it
components values. Specifically this study reveals the inner "hidden" symmetry
of the static flexoelectric tensor that allows minimizing the number of its
independent components to the numbers observed experimentally. Revealed hidden
symmetry leads to nontrivial physical sequences, namely it affects on the upper
limits of the static flexocoupling constants. Also we analyze the dynamic
flexoelectric coupling symmetry and established the upper limits for the
numerical values of its components. These results can help to understand and
quantify the fundamentals of the gradient-type couplings in different solids.
