It's fairly easy to envision. Freezing rain has a warm layer thick enough not too far off the ground to melt snowflakes. Rain simply falls into shallow sub freezing air layer near the surface. Warmth is too much for the melted flake to re-freeze into a sleet ball but freezes on contact with the ground. Sleet has a lot of cold air in all levels except somewhere around 5-9k'. Snow falls above, melts to rain in the middle, and has plenty of cold to traverse through on the way down to freeze into a sleetball. Hope this helps