Configure until what level the items in the structure view should be expanded. The default value is null, which means all items are initially expanded.
Level 0 would result in only the root level items being shown as collapsed: if you load a (DITA) map as your only document, than only the item for this map is initially visible (and collapsed) in the structure view.
Note: if you use level 0, the cursor is also automatically placed in one of the items at level 0 by the editor. So that item at level 0 will always be expanded. This is because of the existing functionality where the outline always expands the path up to and including the item in the structure view that corresponds with your cursor position.
Level 1 in the same scenario would also show the direct children of the root item for the map.
This value affects all instances of the StructureView component. This includes things like the "Outline" sidebar tab provided by this addon, and the project browser modal provided by fontoxml-project-browser or any manually rendered instance.