As the name implies, an Extended License is a Royalty Free license that adds more usage rights over the content --in this case, the photo--. Namely, Extended License adds unlimited copies/reproductions, and most importantly, the right to use the photo in a product for resale --something the standard royalty-free license expressly forbids.  You are going to need an Extended License every time your intended number of copies or digital reproductions exceeds the standard license limit, and, of course, every time you want to use a high-resolution image in a product that you will then resell for profit (like t-shirts, puzzles, canvases, etc.) Extended Licenses come at a higher price point than standard, but their value generally exceeds the cost.