Biostratigraphic units can be distinguished in rocks containing organic remains. They are of the same nature as lithostraton. Both of the stratigraphic subdivisional categories should belong to the same taxonomic system of principal (formal) stratons based on specific characteristics. Regional stratons - stratohorizons - are used for the purposes of stratigraphic unit correlation based on specific characteristics. Stratohorizons are the bodies of rock strata corresponding to the chosen stratigraphic volume fixed in stratotypes, hence they appear to be chronostratigraphic units.-English summary