"Models represent knowledge." - Trygve Reenskaug, 1979 #readingToday
Models represent knowledge. A model could be a single object (rather uninteresting), or it could be some structure of objects.
There should be a one-to-one correspondence between the model and its parts on the one hand, and the represented world as perceived by the owner of the model on the other hand. The nodes of a model should therefore represent an identifiable part of the problem.
The nodes of a model should all be on the same problem level, it is confusing and considered bad form to mix problem-oriented nodes (e.g. calendar appointments) with implementation details (e.g. paragraphs).
(Moon+ Reader Pro v2.3.3, 1979-12-10-MVC.PDF)