Quote from
https://www.tdameritrade.com.hk/en/faqs.html
What is a "pattern day trader"?
Please note that a day trade is considered the opening and closing of the same position within the same day. Per FINRA rules, if you make more than 3 day trades in any 5 business day period, you will be marked as a pattern day trader. Effective September 28, 2001 the NYSE and NASD imposed a
USD$25,000 minimum equity requirement for "pattern day traders." Consistent with the new margin rules, if a TD Ameritrade Hong Kong customer's margin account falls under USD$25,000 and the customer has been marked as a “pattern day trader,” the customer will not be allowed to open new positions until the
USD$25,000 requirement is restored.