Diffractive and non-diffractive collisions are totally different hadronic interaction processes, the diffractive processes are hardly predicted theoretically. This leads to the significant differences in the treatments of diffraction in the hadronic interaction models. Very forward detectors at colliders have unique sensitivity to diffractive processes, and they can be a powerful tool for studying diffractive dissociation by combining them with central detectors. Central information enables classification of the forward productions into diffraction and nondiffraction categories; in particular, most of the surviving events from the selection of diffraction belong to low-mass diffraction events which have not been measured precisely.