The US tax form 1099-B provides transactional information detailing capital gains and losses from disposing of capital assets. At this time, cryptocurrency is classified as property. The information on Form 1099-B helps you fill out Schedule D and Form 8949.
When you sell something for more than it cost you to acquire it, the profit is a capital gain and may be taxable. On the other hand, if you sell something for less than you paid for it, then you may have a capital loss, which you might be able to use to reduce your taxable capital gains or other income.