It's calculates saving percentage between Product Price and the Special Price and displays it on the following pages:

Specials Module
Latest Module
Bestsellers Module
Featured Module
Product Page
Search Results Page (List & Grid views)
Brands/Manufacturers Page (List & Grid views)
Category Page (List & Grid views)
Specials Page (List & Grid views)
Wish List
Product Compare

It'll display text XX% OFF in red round Badge on product image.
You can change text, color & style by altering the in-line css in the xml file.

Tested with OpenCart 2.2.0 - This should work on 2.x.x