Very easy choral music will employ common meters in an easy key. The range will be limited and dynamic markings simple. Notes and rests will be whole, half, and quarter. The tempo will generally be slow to moderate. There will be no polyphony, the voices singing together in unison.
Easy choral music may employ less common meters, as well as eighth notes and dotted quarters. Tempos and keys may change. There may be very simple polyphony, often just two parts instead of four.
Intermediate choral music will make more technical demands in meter, key and range, and offer more variety in dynamic levels and tempo changes. The tempo will range from slow to fast, and there will be four-part polyphony.
Difficult choral music requires more developed technical and interpretative skills, and offers more complex polyphony in a variety of keys and tempos. The text may be in a foreign language.
Very difficult choir music requires advanced technical and interpretive skills. Keys may contain numerous accidentals. Meters may be unusual and switch rapidly. Rhythms may be complex and polyphony may include six or more parts.