The fsteps style is only relevant to 2D plotting. It connects consecutive points with two line segments: the first from (x1,y1) to (x1,y2) and the second from (x1,y2) to (x2,y2). The input column requires are the same as for plot styles `lines` and `points`. The difference between fsteps and steps is that fsteps traces first the change in y and then the change in x. steps traces first the change in x and then the change in y.
See also steps demo.