ISAD(G) is an archival standard supported by UNESCO and the International Council on Archives (ICA). It is a standard for archives produced by corporations, persons, and families. ISAD(G) aims to create consistent and appropriate descriptions for archive material.
It is very similar to EAD. ISAD(G) is the foundation for the descriptions in EAD.
You can read more about the ISAD(G) standard https://archiveshub.jisc.ac.uk/isadg/
ISAD(G) and Vernon CMS
Vernon CMS does not export your data into the ISAD(G) format. However, there is an XML export to the EAD format which uses the ISAD(G) standard.
For more information on how to export into EAD see XML Export and Standards.