Client categorisation is the regulatory classification of a trading client as either "retail" (given the fullest protections, including leverage caps and negative balance protection) or "professional" (eligible for higher leverage and fewer restrictions, in exchange for giving up several of those protections). Qualifying as professional under frameworks like MiFID II typically requires meeting criteria on trading frequency, portfolio size, or relevant financial-sector experience.
Brokers must apply an appropriateness test and can't simply reclassify a client as professional on request — the criteria and process are set by the regulator, not the broker.
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Guide
What to Check in a Client Agreement Before You Sign
The one document that is actually about your account. Which entity, the execution policy, the costs not on the pricing page, and how terms change.