WP All Import - Import Types
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. 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.
This is primarily used to update specific data (e.g., stock/price) for products that already exist (created manually or by other imports). This import type does not keep internal track of the products it imports, so it cannot delete missing products from the file. It also can't maintain the relationship between parent products and their variations, so it can't be used to add variations to existing products.
If you're updating variations with this import type, you should make extra sure that these data points are not being updated in the import settings: https://d.pr/nwqacy.
A "New Items" import can:
A "New Items" import can't:
An "Existing Items" import can:
An "Existing Items" import can't: