The Relativity Designer uses the COBOL FD Importer to import new COBOL FDs from a COBOL source file, or a COBOL syntax file file to a catalog. The import process allows you to selectively add FDs to a catalog; after which, you can augment the file definitions to create your table definitions.
The COBOL FD Importer is also used if you need to update existing definitions in a catalog if your COBOL source definitions have changed. There is also another importer, the Data Source Importer, which you use to merge existing elements from other catalogs into one central catalog.