Est. cash balance is the sum of current cash balances in all currencies. Including available cash and non-available cash.
If your current account generates financing or borrowing, the Est. cash balance will become negative and interest will be charged. If there is short selling in your account, the Est. cash balance will increase and interest will be charged. It should be noted that the interest is generally calculated on a daily basis and settled on a monthly basis, specifically based on the number of financing days. The interest is accumulated daily, deducted uniformly around the 5th of next month, so before deducting interest, the Est. cash balance seen by users is not deducted from interest. If the Est. cash balance before deducting interest is zero, arrears may occur after deduction, that is, the Est. cash balance becomes negative.
The single currency cash amount represents the cash value of the account.
When you open a long position or close a short position in stocks/options/futures denominated in that currency, the cash value will decrease. Conversely, when you close a long position or open a short position on the underlying currency, the cash value will increase.
New deposit will increase the cash value, and withdrawal will reduce the cash value.
Therefore, for a margin account, it's normal to get a negative cash value after you bought a stock or requested a withdrawal. When the cash value is negative, manual repayment is required, otherwise the corresponding currency interest will be generated. The annualized interest rate is detailed on the official website.