Part I handles Section 1256 contracts, combining realised results with the year-end mark on open positions and splitting the total into 60% long and 40% short term. Part II covers losses from straddles and Part III gains from them.
The form is also where the Section 1256 loss carryback election is indicated, allowing a net loss to be carried back three years against prior Section 1256 gains, and where a mixed-straddle-election shows up in practice.
Because futures brokers report an aggregate profit or loss figure rather than individual trades, this form is usually short even for a very active futures trader, which is one of the quiet administrative advantages of section-1256 products.
This is general information for the United States, not tax advice; rules change and depend on your circumstances, so speak to a professional.
Related: section-1256, sixty-forty-tax-treatment, straddle-rules, mixed-straddle-election, schedule-d