A part is a melodic succession of notes that contribute to the harmony and is separate from other melodic successions contributing to the same harmony.
The Bass is the part that performs the lowest notes of the harmony.
When two or more parts exist, the lowest is called the lower part, and the highest is called the upper part. Both of these parts are also called the extreme parts. Others are called the middle or intermediate parts.
The principle part or predominant part is the part with the most melodic interest and is the most striking. This separates it from the other parts called accompaniments or accompaniment parts.
In most elementary exercises, no melody stands out, thus it’s the two extreme parts that stand out to ear the most.
The filling parts are those which only serve to complete or reinforce a harmony already sufficiently indicated by the more essential parts. The Bass is never considered a filling part.
In regards to the number of parts, three are sufficient to produce all constituent sounds of the consonant chord. But for reasons that will be appreciated through practice, the number of parts exceeding the number of notes in a chord is often used. As a result, one part is necessarily strengthened by doubling, tripling, etc., one or more notes of the chord.
Old Music Theory 