Eligibility is narrower than people assume. Typical disqualifiers include being registered with a securities regulator, trading for a business or another person, using the data in a professional capacity, or being employed as an investment adviser.
Brokers must collect an attestation and can be required to reclaim the difference retroactively if the classification was wrong, so a casual misstatement can produce a surprise bill.
Example: a trader pays $10 a month for non-professional depth data. They incorporate and start trading through the company. That usually reclassifies them as professional at, say, $120 a month per feed. Twelve months of unreported use across three feeds is roughly $3,960 in back-billing.
Related: market-data-licensing, exchange-fee, level-2, direct-feed