What's the difference between Commercial Use and Items for Resale?

The biggest differentiating factor is that in Items for Resale, the icon creates the substantial value of the product. 

For example: if you print a book on iconography and use icons throughout the book, the icons make up the value of the book and therefore it is considered an item for Resale. However, if you use icons to designate the sub-section in the book, or to illustrate an infographic, that is considered Commercial Use. In both cases, the book can be available for sale, the biggest determining factor is what is contributing most to its value.

Another example: if you download an icon and use it on the front of the tshirt, that icon is contributing to the success of selling your tshirt and is therefore considered an item for Resale. But if you use the icon on the tag to mark that it can be ironed, that is considered Commercial Use.