1. The dollar amount in an existing margin account that is currently available for purchasing securities. For new accounts, this represents the percentage value of the current balance that is available for future margin purchases.

2. The dollar amount available for withdrawal from an account with existing marginable positions being used as collateral.




Taobiz explains Margin Loan Availability
The margin loan availability will change daily as the value of margin debt (which includes purchased securities) changes, but it may not reflect pending trades that are in between the trade date and the settlement date.

If the margin loan availability amount in an investor's account becomes negative, the investor may be due for a margin call or formal request to sell some of the marginable securities.