Any broker which uses FINRA and Federal Reserve rules related to margin (known as Reg T) as their guide can determine how they will designate an account as a Pattern Day Trader.
Additional Information
Accounts that have not yet been migrated to the intraday margin standards are still subject to the Pattern Day Trading (PDT) rules and are not yet subject to Intraday Margin Deficit (IMD) calls. For more information on the FINRA rule change please review the FAQs on IMD.