The SLS Advanced Boosters are boosters that are a part of the Space Launch System, but can, technically, be attached to any large stage that allows boosters, should other large rockets be added to the game. These and the SLS Solid Boosters are the biggest and most powerful boosters in the game, but can only be attached to the SLS Core Stage.
The SLS Boosters use solid fuel in-game, which makes it impossible to throttle, or to shut down during flight; though they may be jettisoned at any time. Once ignited, they burn at full power then separate from the rocket automatically once all fuel has been exhausted.
The SLS Advanced Boosters differ from the SLS Solid Boosters by shape, paint scheme and they burn longer, but are the same in Lifting Power and Fuel Capacity.