Due to the time required for order confirmation and final settlement, as well as the delayed availability of funds from certain deposit methods (such as checks and ACH transfers), there may be a discrepancy between the funds available for trading and those available for withdrawal.

Effective balance tracks all pending and settled activity in your account and is the truest picture of your net balance when multiple transactions are in motion.

Available to trade reflects only settled deposits in your account as well as the credits and debits from pending trades or withdrawals. So it might be less than Effective, for example, when you are waiting for a check to clear. 

Available to withdraw reflects all settled trades and deposits, minus pending withdrawals. So it might be less than Available to Trade if, for example, you recently sold an investment but it has yet to settle. And it might be similarly less than Effective for pending deposits.