What does "flatdown" mean?

It's referenced in the OPML spec. Here's an explanation:

First, flatdown, in UserLand's outliner terminology, is a direction. The complete list of directions is: up, down, left, right, flatup, flatdown and nodirection. They are used as parameters to the op.xxx verbs which take a direction as a parameter. The first four are structured directions. Going up from an outline element takes you to its previous sibling, down takes you to the next sibling. Left takes you to the parent, right takes you to the first child. The flat directions, flatup and flatdown view the outline as a linear sequence, not structured. Imagine the outline displayed on a screen. To go flatup you'd go to the outline element displayed immediately before, to go flatdown you'd go to the outline element displayed immediately after.