What is the difference between "suite" and "pieces" in imslp.org?

[jimf]

Hi there! I have been using imslp for quite some time and I decided to ask this question here: I was wondering, does anybody know why there are two categories in imslp, one called "suite" and other called "pieces". E.g. Debussy's Children's Corner is called a "suite", while Mussorgsky's Picture at an Exhibition is called "pieces"! (Had I been the one to organize them I would have categorized them the other way round!)
Thanks in advance,
Jim