I am sorry for not clearly stating 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?