Battlefield 4's Second Assault expansion was announced as an Xbox One timed-exclusive a few weeks ago, but EA has now confirmed that it will release on Microsoft's console at launch.
Second Assault contains four "fan favourite" maps from Battlefield 3, remade to take advantage of Battlefield 4's more three-dimensional gameplay and containing "some new multiplayer features." EA's latest press release tells us that it's "available at launch first on Xbox One," effectively meaning that it will release on November 22nd. Other platforms will have to wait for an unspecified length of time - it's usually a month or so.
As you'd expect, Second Assault will be available to buy separately or as part of a Battlefield Premium subscription.