The Snap to Midpoint (SNAP MID) order is similar to a pegged order, as the original order price is determined by the current bid/ask plus or minus an offset. But unlike the pegged order price, the "snap to" order price doesn't continue to peg as the price moves.
A Snap to Midpoint BUY order snaps to the midpoint of the bid/ask minus the offset to set the order price. For a SELL order, it snaps to the midpoint of the bid/ask plus the offset.
To create a Snap to Midpoint Order
Create a buy or sell order.
From the Type field select SNAP MID.
Specify an offset amount that will be subtracted from the midpoint of the bid/ask to determine the limit price.