I have seen it when I have used a Adjustable Trail Stop Order - So usually when I submit a bracket order, I adjust my stop order such that after the price moves a certain amount in my direction, I want to adjust the stop to start trailing. When the trigger price for the adjustable stop order is reached, I see in TWS that my original stop order has changed to a Fix Pegged Order Type. So basically I think its used to implement a Trail Stop order.
Other than this, I myself have not found documentation for this specific order type in their doc.
