Oh, did not know about Custom Fields. Thanks for mentioning it. Now understand and have tested importing EAN numbers several times. Worked perfect. 

it depends on how your plugin stored data. You can ask this from author of plugin. I guess it's using meta fields, so you can import custom fields

Yes, that is why the plugin EAN for WooCommerce has been installed.

As you can see in attachments sent yesterday, EAN is an added field both backstage and visually on the product page.  

Which field to choose (add?) when importing?

Woocommerce has no EAN. It has SKU which is used for any identification

Ср, 4 мая 2022 г. в 17:30, Sizam Support Help Desk <
sizamtheme+EGBRQx8wZQ8Yj2DkJelV@mail.support-hub.io>:

Created a csv file including a column for EAN, but I am not able to find the field for EAN in the Import WP template. Now I add the EAN manually in product data, inventory, and EAN (see attachment). I have installed the plugin EAN for WooCommerce so that EAN can be placed after the short description of the Woo Commerce product (see attachment)

FYI: When exporting all products the header of the column of all EAN numbers is named Meta: _alg_ean

Kindly advice.