The nomeclature involves chronzone, stage, and series. The boundaries of chronostratigraphic units are isochronous surfaces or time lines. The rank of chronostratigraphic units is based on time not thickness.
Geochronometric units are an abstraction.
Several adjacent system comprise an erathem. Phanerozoic eras and erathems are named for major changes in life. The Phanerozoic names are accepted and will not change. There are 2-6 series in system with 3 in most. The stage is basic chronostratigraphic working unit in that it is of a practical size that allows correlation between regions.