I’ve always used Group to simply designate vocal or instrumental although here I found two imaginative suggestions—ISO country codes and keyword tagging—albeit not the most practical for my own library.
So, FWIW: classical heads
Use the Song field to both tag the composer (because that field does not appear on iPhone/Pod/Touch devices) as well as movements and such.
e.g. Beethoven: Trio in B flat, Op. 97: “Archduke” 2nd mvt. Scherzo, allegro
e.g. Bach: “Jesu, Joy of Man’s Desiring,” 10th movement of the cantata Heart and Mouth and Deed and Life, BWV 147
e.g. Brahms: Piano Concerto No. 1 in D minor, Op. 15: b.Adagio
to be continued…