Nobody files this number; it is a leftover. Total open-interest is known, all reportable positions are known, and the difference is attributed to small traders. It is often described as the retail bucket, though small institutions sit there too.
It is popular in folk analysis as a contrary indicator — "small traders are most long at the top" — but the evidence is thin, and the category is a statistical residual that inherits every error in the rest of the report.
Example: open interest of 1,500,000, reportable longs of 1,320,000 means non-reportable longs of 180,000, or 12% of the market. In index futures the share is smaller; in micro contracts it is far larger.
Related: commitments-of-traders, large-trader-reporting, open-interest, micro-futures, disaggregated-cot