How do I combine imports?

Enter import mapping. 

In the "Combine data" panel indicate the field that you want combined for your import and another import. It doesn't have to be the "id" field; you can combine text fields or numeric fields or each field you mapped in this import. 

Once you save the "Mapping" section, return to the list of imports, which should look like this. 

The marked structure shows which of your imports is the main import and which complements its data. You can combine any number of imports this way. If you want e.g. to exclude a large number of products, use Google Docs, create a CSV, or enter the id of the products you want excluded. Then add the created file as the main import and add the product data as the complementary import. You can also upload the created file as a complementary import and later exclude the products labeled with it in the export rule. The whole feature is very flexible and enables any combination of data.