Rock bodies may be classified into many different categories, each of which needs its own distinctive units. The following three categories of formal units are the best known and most widely used:
- Lithostratigraphic units – based on the lithologic properties of the rock bodies.
- Biostratigraphic units – based on the fossil content of the rock bodies.
- Chronostratigraphic units – based on the time of formation of the rock bodies.
For lithostratigraphy and chronostratigraphy, the numerous terms represent different hierarchical ranks; and for biostratigraphy, they are the result of the recognition of various kinds of biozones. Nonlayered igneous and metamorphic rocks, including distinctive terms such as "melange" and "ophiolite", are treated in this Guide under Special Lithostratigraphic Units.
Though lithostratigraphic and biostratigraphic units may be particularly useful in stratigraphic classification under certain conditions or in certain areas or for certain purposes, chronostratigraphic classification is the most useful in terms of worldwide application. Lithostratigraphic and biostratigraphic units are restricted by the limited areal extent of the features chosen to characterize and distinguish them. Few, if any, of these features are both distinctive and present worldwide. Chronostratigraphic units, on the other hand, are based for definition on their time of deposition or formation – a universal property. In principle, they can be recognized the world over to the extent that they are time-diagnostic features distinctive of the unit.