I know that Killing horizon is the hypersurface on which timelike Killing vector field becomes null. Beyond that surface Killing vector field becomes spacelike. But Stationary Limit Surface has also such a property. I wonder, if they are the same thing, if so, why is there different names for this concept even inside the same context(I saw both of them in Carrol's book). If not, what does Killing Horizon means other than that "once crossed the horizon, it is not possible to remain stationary"?