There are a number of other restrictions on placing orders for US stocks, including, but not limited to:
1. When the price for the limit order deviates more than 40% from the market price, the FUTU risk control system will reject the order.
2. A fixed price is needed to place an order for a small number of shares, such as multiples of 0.05 or 0.5.
3. Orders that meet the FUTU risk control rules will be submitted to upstream, but may also be rejected because they do not meet the upstream risk control rules. Common reasons such as excessive price deviation, temporary not support for trading, fail to find the corresponding stock symbol, etc.
4. Market orders will be submitted to the exchange during extended trading hours, but may not be able to be executed.
5. The upstream brokerage defines an order of less than 100 shares as a fraction order.
6. Orders submitted during the matching period may be submitted to non-traditional exchanges.