There are two choices available when selecting "Allow Outside Regular Trading Hours" for a stock Stop Limit order or a Trailing Stop Limit order. The first choice is for "Allow this order to be triggered or filled outside of regular trading hours" and the second choice is for "Allow order to be filled during pre-open session".
If you select "Allow this order to be triggered or filled outside of regular trading hours" then the order can be triggered and filled during the pre and post-market sessions. However if you select "Allow order to be filled during pre-open session" then the order will not fill during the pre-open session if the order has not already been triggered. However, please note that under both selections the order will still work during regular trading hours (RTH).