Specifies the global break condition for the electronic SC-loop. The relaxation of the electronic degrees of freedom will be stopped if the total (free) energy change and the band structure energy change ('change of eigenvalues') between two steps are both smaller than EDIFF. For EDIFF=0, NELM electronic SC-steps will always be performed.
Mind: In most cases the convergence speed is exponential. So if you want the total energy significant to 4 figures set EDIFF=. There is no real reason to use a much smaller number.