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.