If this payment method returns to the store via a silent post or by not actually returning the customer's browser directly to the store, checking this field will cause the order stored in TEMP_Sessions to be added to the session started by the payment method (in $SC['order']), so Squirrelcart can complete the order. 

This will happen when $_GET['cc_return'] == 4 AND the actual session name variable is passed by the payment method (or set to $_GET['PHPSESSID']  from another variable in squirrelcart/payment_gateways/gateway_name_pre.php) and is set to the customer's session ID.