[KevinH]

Many times early epub creators used nbsp in p tags and br to physically create whitespace to force a layout when using css is the more correct way to handle this. Things like space above, line spacing, and space below an image or paragraph are better set by using css so that screen readers for visually impaired people are not confused. The rule is "Keep Layout (css) and Content (text) separate".

That paragraph in the user guide is pointing out how you could use find and replace to "fix" these epubs that look like they came from the typewriter age of layout.