Diff. Specific Layers
A diff. specific layer is written to .osu files rather than the mapset's .osb.
On the layer tab, click this button to make the layer diff. specific.
You'll notice the layer moved to the top and can't be moved below .osb layers. This is because storyboard elements are loaded first from .osu, and then from .osb.
To move this layer back to where it was relative to the other layers, You have to use osu's Background and Foreground layers like this.
By default, storybrew only exports to the .osb file and to the current .osu map opened. You can right click the export button to make it go over every map of the mapset and export the diff. specific layers into each .osu file.