two parts: the two parts in solfeggio refer to the melody, timbre and vocal area, and the coordination of two different or the same parts. Is the high voice and low voice. Generally, the high-pitched part is a melodic part.

Voice: Speaking of voice, we should first say "multi-voice music"

In the music we usually hear, we can not only feel its main theme, but also hear other voices of other instruments taking other tones. Music like this is multi-part music. It can be said that cantata with accompaniment, or at least two people's chorus (non-chorus) is the smallest multi-part music.

then, it is not difficult to understand what the voice is, that is, it refers to the respective tones played by each sound category or each group of instruments including the main melody.

Classification method of voices:

Usually pitch is classified by timbre or musical instrument!

as for the difference you said, you should almost know it after you read it! I hope my answer can help you!