All supported Geosoft spatial datasets, such as databases, maps, grids, voxels, etc., maintain metadata that describes the data. The metadata is stored as XML in a file next to the data file or among the set of files for file types that use file sets. The metadata XML file will have the name of the main dataset file with '.xml' appended. For example, a grid file named 'mag.grd' will have a metadata XML file named 'mag.grd.xml'.
Geosoft created metadata will conform to ISO 19139 (https://www.iso.org/standard/32557.html) or FGDC (https://www.fgdc.gov/metadata). ISO 19139 is the Geosoft default. Organizations that work under the FGDC standard will have a custom metadata editor and controls that ensure FGDC compliance. Geosoft maintains Geosoft specific metadata in a node tagged "geosoft" that will be present inside the "<gmd:MD_Metadata>" tag for ISO metadata and inside the "<metadata>" tag for FGDC metadata.
Refer to http://www.geosoft.com/schema/geo, which describes schema tags in more detail.
See also https://github.com/GeosoftInc/gxpy/blob/master/reference/SingleDataset.metadataeditor_config, which provides a comprehensive list of permissible entries for various settings.