Hi @awwe ,
Thank you for appreciating our efforts. Please feel free to rate us or buy a coffee to support the development.
The product code is only supported by simple product and it will only visible when you add the field is only visible when you input a value in this field. When you activate the plugin, all those already published products will also be integrated with the field but unless you provide a code to the field, no data or field will be visible.
Editing or updating the product will not make you lose the field data but if you change the product type or other necessary product attributes, then the field can be unavailable for the updated product.
Thanks.