Every fcm reports position data for accounts that exceed a product's reportable level. This dataset is how the cftc monitors concentration, enforces position-limits, and produces the weekly commitments-of-traders report.
It also means large positions are never anonymous to regulators, even when they are invisible to the market. Traders below the threshold sit in the "non-reportable" bucket in published data.
Example: reportable levels are commonly a few hundred contracts. A trader holding 25 ES contracts is non-reportable; a fund holding 5,000 is reported daily and shows up in aggregate COT categories.
Related: position-limits, cftc, fcm, accountability-level