GSSAs are defined by the International Commission on Stratigraphy (ICS) under the auspices of their parent organization, the International Union of Geological Sciences (IUGS), and are used primarily for time dating of rock layers older than 630 million years ago, lacking a good fossil record.
This is because the Earth's crust in geological time scales is constantly being recycled by tectonic and weathering forces, and older rocks and especially readily accessible exposed strata that can act as a time calibration are rare.
In contrast to GSSAs, GSSPs are based on important events and transitions within a particular stratigraphic section.
In older sections, there is insufficient fossil record or well preserved sections to identify the key events necessary for a GSSP, so GSSAs are defined based on fixed dates and selected criteria.
The ICS first attempts to meet the standards of the GSSPs (see below) and if those fail, usually have enough information to make a preliminary selection of several competing GSSA prospects or proposals.