piān 篇 is a counting unit in Chinese works. It can refer to a part of text, thus meaning 'section' or 'chapter', but can also mean 'article' or 'essay'.
Pian can consist of several juan 卷, but juan can not consist of pian. If a pian consists of juan than juan usually means 'chapter', while pian would then mean something like 'volume', or 'article'.