A categorical distinction is made between two types of biostratigraphic units. In a given superposed section, a eubiostratigraphic unit is a thickness of strata whose boundaries are defined by the lowest and/or highest occurrences of fossils of one or more specified taxa. In contrast, a quasibiostratigraphic unit is a thickness of strata characterized by its fossil content, but whose boundaries are defined by other means.

Eubiostratigraphic units include range, single-taxon interval, assemblage, and multiple-taxon interval fossilzones. Quasibiostratigraphic units include paleontologically distinct lithozones, metrizones, and “fuzzy” zones. Quasibiostratigraphic units serve to summarize the most distinctive changes in overall fossil content observed in individual sections, without necessary regard for the exact vertical ranges of particular taxa. Most quasibiostratigraphic units need not be given formal binomial names, but are best assigned alpha-numeric or informal descriptive designations.

Assemblage fossilzones are multiple-taxon analogs of range fossilzones, in that they are defined by a single set of two or more taxa. Multiple-taxon interval fossilzones are multiple-taxon analogs of single-taxon interval fossilzones, in that their boundaries are defined by two different sets of two or more taxa. As such, multiple-taxon interval fossilzones are stratigraphically and temporally contiguous with one another by definition.