A note is the node type of just about any reply on PM. While the root of a thread may have various nodetypes, the replies will all be "notes". The only exception i can think of is for patches whose reply types are a user restricted type of pmdevnote. Oh, maybe the C&A stuff as well. Either way the point is to generalize the behaviour of replies on the site.

This is probably the main reason why Newest Nodes displays all the replies in a single big list and not under the section headers as the root nodes are placed.