There are many reasons an amount may need to be adjusted. The IRS mandates specific adjustment codes for the reporting of capital gain/loss transactions. These adjustment codes are listed starting on page 7 of the Instructions for Form 8949 Sales and Other Dispositions of Capital Assets, along with information explaining the situation each code represents and information regarding how to properly report the adjustment amount in your return.
These adjustment codes are included on Form 8949, which will print along with Schedule D (Form 1040) Capital Gains and Losses. The adjustment amount will also be listed on Form 8949 and will transfer over to Schedule D. For more information, see the IRS Instructions for Schedule D.