The constraint
A margin account under $25,000 in equity is limited to three day trades in a rolling five business days. A fourth restricts the account. This is not a guideline that an automated strategy can be tuned around — it is enforced by the broker.
What it means for design
Under that threshold, high-frequency intraday equity strategies are unavailable regardless of merit. Swing-holding strategies, cash accounts with settlement constraints, and futures or crypto — which are not subject to the rule — are the structural alternatives.
The design lesson
Account structure belongs in strategy selection at the beginning, not as an obstacle discovered after building something that cannot legally run. This is the cheapest constraint on this page to respect and among the most common to overlook.