Export and Import Summary
You use the File > Open Catalog command to load individual catalogs. Lightroom can run only one catalog at a time. The main thing to remember is that a catalog export is always a one-way process. You can’t get a catalog export to add to an existing catalog—you can only create new catalogs.
The File > Import from Catalog command is the mechanism used to import catalog information from a catalog or add new catalog information to an existing catalog. You also can use this command to update the metadata information in the current catalog.