The Collection column is the only column that you can add to the CSV file that doesn't break the format. You can't add any other columns to your CSV file. To organize your products into collections during the CSV file upload, you can add a new column anywhere in your CSV file with the header name Collection.
I highly suggest downloading a sample product CSV file to use as a template to ensure you don't run into any issues.
Let me know how you get on!
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