'Duration' describes how the BullionVault exchange will complete your order.
Your 'permanent' order is matched as far as it can be, and what's left over is posted on the public order board until either it's completed, or it's cancelled by you, which is why it's sometimes called a 'good until cancelled' order.
Your 'immediate' order attempts to do what it can now within your limit, and abandons the rest without posting it on the order board.
A 'fill or kill' must complete in entirety, or be abandoned in entirety, so it never part executes.
The final option works like a permanent order but sets a time limit which says when the remnant of your order should be withdrawn by us from the board if it is still unexecuted. This is sometimes called a 'Good until' order.