When you export a list of order items from an order, the system generates a CSV file.
When opened with a spreadsheet editing program, the CSV looks like this:

Important: The notice on line 1 references the column headings on line 4, the "--Qty--" and "--Variants--" entries. These entries are invalid, and must be modified by you before the CSV file is ready for use in the Mass Create tool.
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The value '--Qty--' is invalid, and must be changed to one of the following two values:

QtyIf you use "Qty" as the column heading, the Mass Create tool assumes you are setting the quantity for each line item in the CSV. In our example file, Mass Create would set the quantity of Retreat to Coralhelm - Foil (Light Play, English) to 2.Add Qtyif you use the value "Add Qty" for the column heading, the Mass Create tool will adjust the current quantity for each line item in the CSV. In our example file, the Mass create tool would add 2 to the quantity of Retreat to Coralhelm - Foil (Light Play, English). Using "Add Qty" with positive numbers (like '2' in our example file) has the effect of adding the items in the current CSV back into stock. However, if you use negative numbers in the "Add Qty" column, that would remove the quantity from stock. If we were using the changed our example file to '-2' on line 5, Mass Create would remove 2 quantity from the existing stock in  Retreat to Coralhelm - Foil (Light Play, English).

Because variants are not consistent from one product type to another, and because orders may contain items from mixed product types, it is not possible to export a single CSV with perfectly structured variant data. This area of the CSV will display the variant values that are present for each line in the CSV, but it will be up to you to name the columns correctly. For reference, the most common variant is "Condition," and many product types also have a "Language" variant as well. In our example, the export contains only products from Magic Singles, so I would change the placeholder to "Condition" and add a new value "Language" in the next column. The final version of line 4 would look like:Product Name, Category, Qty, Condition, Language
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The top three lines of the CSV are there as an aid, but they are invalid for use in the Mass Create tool. Make sure to remove these rows before you save your final CSV.