Treeitem.setOpen/isOpen vs. Treemodel.addOpenPath(int[])

Hi, we have very large trees (about 100k nodes / paths) with multiple columns (up to ~20 or more). In these trees we get into serious performance troubles when using the "naive" tree model. We therefore implemented our own tree model(s) that add and remove nodes as their parents are expanded or collapsed. However, what puzzles me all the time is the question "What is the difference between setOpen and addOpenPath?" IMHO the javadoc is not very helpful. What I need is a concise formal definition of what these are and what to call when, no generic description like "adds the path to the open" (whatever that may be).