The SLS Boosters are boosters that are officially part of the Space Launch System, but can, technically, be attached to any large stage that allows boosters. These 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. Once ignited, it burns at full power until the fuel is exhausted, and separates from the rocket automatically.
The SLS Solid Boosters differ from the SLS Advanced Boosters by shape and paint scheme, but are the same in Lifting Power and Fuel Capacity.