- New Items Import -
This is primarily used to create and manage products. It keeps internal track of the products that it imports, which means that it can later update/create/delete products as they're changed/added/removed in your import file (see: http://www.wpallimport.com/documentation/recurring/update-import). It's also the only import type that can add/remove variations for variable products.
One limitation of new items imports is that cannot detect/update products that it didn't previously create. If you need to use a "New Items" import with a file that contains existing products, you can use WP All Import's API to prevent duplicates from being imported: https://www.wpallimport.com/documentation/developers/code-snippets/#do-not-create-products-with-duplicate-sku.
- Existing Items Import -