You can include the following columns:
1) UPC, EAN, or ASIN (required)
2) Cost (optional)
3) Supplier SKU (optional)
4) Supplier Title (optional)
5) Custom 1
6) Custom 2
7) Custom 3
Therefore, your Column A should be the UPC Code. Column B the Cost. Column C the supplier Sku - at a minimum to get the best returns from the tool.
If you put the Supplier Title in Column D, then when you get the resultant file, it will have both the Amazon title and the suppliers title/description so that you can begin to ferret out any ASIN mismatches. This is incredibly important if you are doing any dropshipping.