Maintenance Margin is set by the exchange. This is the amount required to carry a contract past the daily close.
DayTrade Margin is set by AMP. This is the amount required to enter into a position per contract on an intraday basis. These margins are in effect anytime the market is open, except the last 5 minutes of each trading session.
AMP request that you either flatten open positions or meet the exchange required maintenance margin during this time period.
Once the Market Re-Opens (Over-Night Trading Session), margins are back to day trade margin requirements until the next daily close.