This vqmod will replace the default sort method (product's Sort_Order field) on the Category page.
This mod will sort the products based on their quantity, on the stock. So this way the products that are is stock will always be displayed first. IF you have products with the same quantity, then those will be ordered by Name.
This works with pagination, so if you have 4 pages, page 4 will display the Out Of Stock items.
Listing like this is useful if you want to sell products that are in stock.