iTunes apps tab, list view, what does the 4 symbol mean?

In iTunes, in the Library section on the left, with the Apps category selected, I see all my downloaded apps as icons. When I choose List from the iteims across the top, the view changes to an apphabetical list. Next to the name of each app is a symbol - a rectangle containing a numberal and a + sign. Most of my apps show 4+, others 12+, and a few 17+. Can't figure out what the symbol means. If you know, I'd be grateful for an explanation. Thanks.