“Shuffled” is the old way - everything in random order.
“Ascending level then subject” is the lessons first ordered by level (i.e. level 7 lessons before level 8 lessons), then by type (radical, kanji, vocab).
… looks like @ccookf beat me to it… lol
Also, if you want to level as fast as possible, I would recommend using a re-order script to just order by type (radicals first, then kanji, then vocab). If you just use the “ascending level then shuffled” option, it actually won’t allow you to level up in sevenish days doing 20 items a day, because you get all the previous levels vocab first, when you really should be doing radicals and kanjis first (kanjis can actually be done within the first few days and still maintain a fast leveling pace).
If you don’t want to go as fast as possible, it’s mostly personal preference… some people don’t like getting all the kanji at once…