The Margin used represents the amount you have utilized for any open positions that gets deducted from your cash balance in your trading account.
Lets assume you start your day with no positions in your trading account.
You have a Holding of 50 shares of SBI in your Demat account.
The LTP of SBI is 245 and you decide to sell your 50 SBI shares at this price.
Your trading account will be credited with Rs.12,250.
Total Account Value = Margin Available + Margin Used
Margin Available = Total Account Value - Margin Used
Lets say your Total Account Value currently is Rs.20,000.
Now when you sell shares and the amount gets credited to your account, the margin used will show in negative because according to the equation, the amount has to be added to the Margin Available.
Margin Available = 20,000 - ( - 12,250) = 20,000 + 12,250 = Rs.32,250