You have mentioned that "either" used or carried over soon after "You don't get to pick/choose".
I am sorry for not clearly stating the the question which let me re-phrase:
Let us ignore and keep aside tax efficiency of the choice/decision and amount of taxable income a person has. And let us assume there is no previous carried forward cap loss and 2019 is the first year of cap loss consisting of $2,000 short term cap loss and $6,000 long term cap loss = Total $8,000 Cap Loss.
Is $3,000 cap loss offset in 2019 in line 6 of 1040 a matter of choice or is mandatory?