The overnight margin is set by the exchange. This is the amount required to carry a contract past the daily close.
The day margin is set by AvaFutures. This is the amount required to enter into a position per contract on an intraday basis.
The day margin is in effect anytime the market is open, except for the last 15 minutes of each trading session.
AvaFutures requests that you either flatten open positions or meet the exchange required overnight margin during this time period.
Once the market reopens (overnight trading session), margins are back to day trade margin requirements until the next daily close.
If you do not meet the required overnight margin on a trade, and AvaFutures is forced to close your trade, you will be charged $10 for that action.

For example, if I open 1 contract trade on GC Futures (Comex Gold), the overnight margin for that trade will be $7,800.
That means I must have available in my equity, >$7,800 in order to maintain the trade margin through the overnight session.
Once the day session begins again, my margin will drop to the day rate margin of $1,716.
You can find the requirements for the day margin and overnight margin of each instrument here.