What does "Is Document" includes?

I have a scope that I think I need to change after adding the "Link to Document" Content Type. Now I am adding 5 new rules to the scope where I will be using ContentType="so and so" with Include.

Now the scope still has "Is Document" and I can't find an item that has "Link to Document" content type. But the item is crawled because if I select "All Sites" from scope drop down I get the file.

What does "Is Document" emcompases (file type wise: does it include all office file, media, jpg, tiff, pdf, Autocad file, etc.) ?