Reference -- Import Records
The imported metadata must be formatted as either an mdEditor file, mdJSON file, or FGDC CSDGM file. mdEditor and mdJSON formats have the capacity to contain multiple metadata records per file, FGDC CSDGM can support only a single metadata record per file.
During the import process you can choose which metadata records, contacts, and/or dictionaries within the import file you wish to load into browser cache. You also have the option to merge imported records with the currently loaded records or replace them.