Dielectric relaxations in ferroelectric ceramics occur at various frequencies, depending on the type of chemical or physical defects. These defects depend on either intrinsic or extrinsic heterogeneities due to special heat treatments (quenching, annealing,…), ionic substitutions, grain size additives, and grain boundary nature. The value of the relaxation frequency f_r increases from about 10² to 10¹² Hz as the scale of the defect phenomenon decreases from microstructure, to nanostructure, to unit-cell, to atomic vibrations. This type of study requires a pluridisciplinary approach involving solid state chemistry, materials science, solid state physics and various industrial aspects. The applications in the area of electronic ceramics are related to the value of f_r. In absorbants, the usable frequency is close to f_r, whereas it is far from f_r in good insulators. In view of numerous experimental examples, a classification is proposed to predict which materials may be suitable for a given application.