On the first day of every month SixthContinent resets the credit wallets of all the inactive users, and redistributes these credits through a special reward for the top spender members: The ExtraProfit.
The ExtraProfit is a reward for those members who spend $300 (or multiples of $300) within a month.
$300 spent in July, you earn 1 ExtraProfit;
$600 spent in July, you earn 2 ExtraProfit.
So if you want to earn lots of extra credits every month, reach the threshold to obtain one or more ExtraProfits.