if you have a net capital loss line 16 of schedule D. you have the option of carrying back some or all of those future losses to 2021 by checking box D on the 6781. the computation of how much can be carried back is a little complicated. it depends on having both net capital gains and net gains on form 6781 in those prior years. you can read the instructions under item D on the form
https://www.irs.gov/pub/irs-pdf/f6781.pdf
