See it like this discretionary income is that constant flow of money you have after your expenses( it can be in a monthly or a yearly basis), while discretionary funds is the actual cash from that flow that you can make use of at the moment.

Discretionary Income and Funds may have little differences but it is not enough reason to say that they are completely two different things this is because Discretionary Income generates Discretionary Funds, meaning that when you keep receiving that extra income over time and don’t immediately spend it, it accumulates into your Discretionary funds.