It's a Z fare (which you can see on the checkout page). Further, if you click on the fare rules link, you will see it is specifically a ZL5L76D7 fare (there are multiple Z fare filings). Z is the lowest fare class and this is currently the lowest Z fare filing as it is fairly heavily restricted. In addition to below, it also requires a Saturday night stay or 7-day min stay. As you can see, it only available if your outbound flight is on May 8th or earlier.
.DAY/TIME APPLICATION
OUTBOUND - PERMITTED MON THROUGH THU ON EACH TRANSATLANTIC SECTOR. INBOUND - PERMITTED SUN THROUGH THU ON EACH TRANSATLANTIC SECTOR.
SEASONALITY
PERMITTED 01APR THROUGH 08MAY ON THE OUTBOUND TRANSATLANTIC SECTOR.