The simple mode only requires the counterpart/pair of DFI to be added. The system would then automatically swap half of it into DFI, and you would end up having 50/50 of each coin in the pool. While in the normal mode, it would be required that you already have DFI available, and it that case you can choose yourself how much you would like to add in the pool, as long as you have enough coins on each side.
To know more about Liquidity Mining, click here.
To know how to remove liquidity, click here: How to withdraw from Liquidity Mining / Remove Liquidity?