The OPML (outline processor markup language spec) uses the term flatdown. What does it mean?
expansionState is a comma-separated list of line numbers that are expanded. The line numbers in the list tell you which headlines to expand. The order is important. For each element in the list, X, starting at the first summit, navigate flatdown X times and expand. Repeat for each element in the list.
According to http://www.scripting.com/2007/07.html:
"It appears in the definition of the expansionState element, which is an element specifically for people who are implementing outliners, and those people surely know what flatdown means (informally, it means moving to the next node down from where you are, regardless of structure). But even an implementer of an outliner could ignore expansionState and all that would happen is that the user would have to re-expand the outline as he or she likes it. It's a convenience for the user, basically. Certainly not crucial to anyone's implementation of OPML."