Under the European framework a client can elect to be treated as professional by satisfying two of three tests: a record of significant trades at meaningful size and frequency over the past year, a financial instrument portfolio above a set threshold, and at least a year of relevant professional experience in the financial sector. The firm must also assess competence.
Electing changes what you give up as much as what you gain. Leverage caps under esma-leverage-caps no longer apply, but neither do the standardised risk warnings, the guaranteed negative-balance-protection in most implementations, and in many cases access to the investor-compensation-scheme and the ombudsman service.
Brokers promote the category because professional clients trade larger. The thresholds and consequences vary by jurisdiction, and the client agreement rather than the marketing page is where the actual terms are set out.
Example: a professional-category client can be offered 100:1 on a major pair where a retail client is capped at 30:1, so the same EUR 100,000 notional needs EUR 1,000 rather than EUR 3,333 of margin.
Related: esma-leverage-caps, negative-balance-protection, investor-compensation-scheme, fca