Buying power, margin requirement, and liquidation price for leveraged positions.
A margin call occurs when your account equity falls below the maintenance margin requirement. At that point, you must deposit additional funds or the broker will liquidate your positions.
The liquidation price is the stock price at which your equity equals the maintenance requirement. Trading on margin amplifies both gains and losses proportionally to leverage. Never use full buying power — maintain a buffer above maintenance margin to absorb volatility.