Existing Categories replaced by other categories after importing new categories
Commerce categories are used to create a hierarchical structure of items in your web store. The fields supported for commerce category CSV import reflect those on the Commerce Category Record at Commerce > Content Management > Catalog & Categories > Commerce Categories. After the import, the categories and items are displayed on your site and are available for management through Site Management Tools and also through the Commerce Category record in NetSuite.
Each CSV file includes an External ID column. Use the external ID in your CSV files to link the rows in one file to the relevant row in another file. You should also use the external ID for columns that reference a subcategory. Even though you can use the category names in these columns, category names are not necessarily unique and it is best practice to use the External ID instead
If there are some already existing commerce categories in the Netsuite account and for the next level import we should aware of the existing categories. so it is essential to take a backup of existing commerce categories so that after the import of the next step of categories we can check whether it affects the existing categories. some existing categories may get replaced by new categories in some situations because of using the same external id that we used for the import before. so make sure the external id used for import does not replace any existing categories during import.