Attach a picture and the model starts from it instead of from nothing. There are four ways it can be used, and they are not variations on one idea.
Each one has its own page, because each answers a different question:
redraws the whole picture, keeping as much of it as you allow.
redraws only where you paint, and keeps what was under the brush as its starting point.
also redraws only where you paint, but throws away what was there first.
grows the picture outwards and fills in the new space.
blends your picture with several others, and only Stable Cascade models serve it.
ControlNet also reads an attached picture, and it has a page of its own because it never redraws your picture. A detector reads the structure out of it (an outline, a depth map, a pose), and the model paints a new picture that follows that structure.Masked work of any kind needs a Stable Diffusion 1.5, 2 or XL model. On newer model families the mask is ignored and the whole picture is redrawn. The panel warns you before you spend kudos on it.