Used margin is the sum total of margin used for all open positions. Suppose you have three positions open, with margins of $300, $500 and $900. Your used margin will be $300 + $500 + $900 = $1,700.
The initial amount of used margin for a position depends on the leverage used. A $300 dollar long position opened at 3:1 leverage will use $100 margin. A $300 position opened at 2:1 leverage will use $150 margin. A $300 position opened at 1:1 leverage will use $300 margin.