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Candidate GSSPs [and likewise GSSAs] are evaluated by the ICS and its constituent subcommissions and working groups based on a long list of criteria (Hedberg, 1976; Cowie et al., 1986; Salvador, 1994; Remané et al., 1996; Harper et al., 2023). The most important of these is that the boundary at the candidate stratotype is defined at the level in rock coincident with 'a single stratigraphic signal within an interval of multiple, varied stratigraphic signals', that should allow for reliable, high-resolution correlation across the greatest possible palaeogeographical range of palaeoenvironmental settings (Finney, 2013).

Identification and field investigations of candidate stratotype sections and boundary intervals, and drafting of proposals, are carried out by subcommissions of ICS (stratigraphy.org/subcommissions) and usually by smaller boundary-working groups. One or more candidate GSSP proposals [and/or GSSA proposals] may be considered for approval by the voting members (ca. 20; voting member tenured for a maximum of 12 years) of a subcommission, and a single proposal that receives a supermajority vote (>60% of the voting membership) is considered approved. One consequence of this approach is that disagreement can arise, because type sections that are favoured for historical reasons may be abandoned, previously established boundary levels may be greatly changed, and in some instances historical units (and their names) are replaced by new and different ones (retiring old names).

Onceaccepted by a subcommission, a proposal is then forwarded to the ICS for consideration and approval. The ICS voting members are the executive officers of ICS and the chairs of the 17 ICS subcommissions, each of whom has a single vote. They evaluate and discuss each proposal, then vote. If the proposal is approved by supermajority votes, the proposal is forwarded to the IUGS Executive Committee. Once it is also approved by majority within the IUGS Executive Committee, the proposal isformally ratified and recognised as an international geostandard. The ICS executive updates the International Chronostratigraphic Chart and the GSSP table onstratigraphy.org (typically within days).

A manuscript documenting the ratified GSSP proposal is submitted for publication, generally in the IUGS's journalEpisodes (episodes.org). At the GSSP stratotype locality, a physical marker should be placed, that may be officially launched in a public dedication ceremony (typically 1-3 year after ratification). A proposal to change a newly established unit and/or GSSP cannot be made within the first 10 years after its ratification, a clause to ensure the stability of the global chronostratigraphic framework.

Cowie, J.W., Ziegler, W., Boucot, A.J., Bassett, M.G., and Remané, J., 1986, Guidelines and Statutes of the International Commission on Stratigraphy (ICS). Courier Forschungsinstitut Senckenberg, v. 83, pp. 1-14.

Finney, S.C., 2013, The Reality of GSSPs. Ciências da Terra, v. 18, pp. 9-12.

Harper, D.A.T., Bown, P., and Coe, A., 2022, Chronostratigraphy: understanding rocks and time. In: Coe, A.L., ed., Deciphering Earth's History: the Practice of Stratigraphy. Geological Society, London, pp. 227-243.

Hedberg, H.D., ed, 1976, International Stratigraphic Guide - A Guide to Stratigraphic Classification, Terminology, and Procedure. New York, John Wiley & Sons, 200 p.

Remané, J., Bassett, M.G., Cowie, J.W., Gohrbandt, K.H., Lane, H.R., Michelsen, O., and Wang, N., 1996, Revised guidelines for the establishment of global chronostratigraphic standards by the International Commission on Stratigraphy (ICS). Episodes, v. 19, pp. 77-81.https://doi.org/10.18814/epiiugs/1996/v19i3/007

Salvador, A., ed, 1994, International Stratigraphic Guide - A Guide to Stratigraphic Classification, Terminology, and Procedure, Second Edition. Boulder, Colorado, The International Union of Geological Sciences and The Geological Society of America, 214 p.

A geologic section has to fulfill a set of criteria to be adapted as a GSSP by the ICS. The following list summarizes the criteria where the imperative wordsSHOULD andMUST are defined according toRCF2119:

1. A GSSP sectionMUST contain a stratigraphic marker that recognizes the lower boundary of a geologic Stage
2. The boundaryMUST be recognized by an observable, unambiguous change in the physical properties or fossil content of the strata, for example the first appearance datum of a fossil species
   • ThereSHOULD be other physical manifestations (secondary markers) that can be used to locate the boundary, e.g. other fossils, chemical changes, geomagnetic reversal
   • The section in which the marker appearsSHOULD have layers containing minerals that can be radiometrically dated to assign ages to the depositional events at or bracketing the boundary horizon
   • The primary or secondary marker eventsMUST be manifested in regional and global stratigraphic sections to allow correlation in outcrops of the same age
   • The markers should be independent of facies
3. The outcropping stratigraphic section containing the GSSP markerMUST have adequate thickness to support global correlation
4. Sedimentation in the section containing the boundary mustMUST be continuous without any changes in facies
5. The section containing the markerSHOULD be unaffected by tectonic and sedimentary movements, and metamorphism
6. The section containing the marker mustMUST be accessible to research and free to access
   • The section containing the markerSHOULD be located where it can be visited quickly andSHOULD be kept in good condition (ideally a national reserve), in accessible terrain, extensive enough to allow repeated sampling and open to researchers of all nationalities
7. The fossil content in the boundary interval SHOULD be abundant, distinctive, well preserved, and represent a fauna and/or flora as cosmopolitan and as diverse as possible.
8. In order to ensure its acceptance and use in the Earth sciences, a boundary stratotype SHOULD contain as many specific marker horizons or other attributes as possible favorable for long-distance correlation.