I'm not sure what you mean by "supplemental form". Forms 1099-B don't always have the correct cost basis. This happens for employer stock sales such as Restricted Stock, Non Qualified Stock Options, Employee Stock Purchase Plans. The 1099-B sometimes is accompanied by a supplemental schedule that shows the correct cost basis. In cases where the stock basis is incorrect on the 1099-B, the cost basis must be adjusted after the amount on the Form 1099-B is entered. If you don't understand what is reported on the supplemental form, I suggest you contact your broker and inquire about it.
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