What do "Menu > Layer > Convert to Drawing" do exactly?

What do “Menu > Layer > Convert to Drawing” do exactly?

It doesn’t say much in the reference guide
http://docs.toonboom.com/help/storyboard-pro-5/storyboard/reference/menus/main/layer-menu.html

I understand what convert to vector and convert to bitmap is.
But what is convert to drawing? What is a drawing? Isn’t it all drawings?

Best regards Mads