This EditMode allows you to add some detail to maps via billboards. These are mostly used to add light shafts, god rays and the like, but should work for doing other kinds too.
To create a Billboard, just click on the grid when this EditMode is active. Optionally, there are two parameters one can set up prior to placing Billboards on maps. These are:
Have into account that these options will be valid for objects created right after changing them, so any Billboard that is already created will keep its settings.
More on Billboards here.