When purchasing an item, the buyer is in fact purchasing authorization to make use of that item. Each license allows the buyer to use the item purchased in a specific way. Here is a straightforward explanation of what each license allows you to do so that you can make the right choice.
With a regular license you can use the item purchased in your own project or in a project that you complete for a client, whether it is a website, an ad, a presentation, a video clip, or a free app. The item can only appear on one domain, one subdomain or one directory.
What you cannot do with the item purchased is resell it either as part of a project or on its own. You can create a website incorporating the item, but you cannot use it to create a website template for sale. You can use it to create a free mobile app, but you cannot incorporate it into an app that you offer for sale..
The extended license lets you incorporate the items purchased into projects you undertake with the intention of selling them. These might include a t-shirt design, or a website template, or a dvd release.
You cannot resell the item purchased independently, only as part of a larger project that you wish to sell. The item purchased under an extended license can appear on multiple domains, subdomains and subdirectories. Property of the original item will always belong to the author.