Until now you could only add / subtract / intersect the previous results with the new results within the same playlist.
Now you can add / subtract / intersect any of the existing smart playlists. You can have a smart playlist just to mix different lists into one (just fusion rules, no other rules), or you can mix the results of some rules with the results of other lists.
By simply tapping on "A", "B", "C", "D", "E" or "F" you cycle through the set operations "+" (union), "-" (minus) or "=" (intersection).
Adding is done first, then all subtractions are done, finally all intersections are executed.
In the above example for smart playlist "E", all songs with BPM < 160 are selected in the iPod music library, then these results are joined with the current results in list "B" and "D" and then this result set is intersected with list "A".