The measurements of susceptibility (dielectric, elastic) with very small amplitudes of the conjugated measurement fields allow to study properties and applications. With crystals simultaneously ferroelectric and ferroelastic, different situations have been experimentally studied: quasi continuum medium near some transitions, metastable domain walls in strong interaction, isolated domain walls whose effects add up, ionic modifications inside the wall, even motionless. The maxima of losses often allow to delimit the different situations. These phenomena can coexist. Some of them may be negligeable in certain experimental situations of a certain crystal, but searching and studying them may be enlightening.