What is the difference between Extended License & Regular License?

In simpler words –

Extended License gives permission to use the plugin / theme in one product like SaaS, where you install zip, and a lot of users use it, and you can charge them for that “feature”, but users don’t have access to the source files. Example – You’ve developed a blogging platform (may be like Medium.com) using WordPress Multisite Network where users don’t have access to the code. If you want to offer functionality of our plugin to your users, Extended License is required.

Whereas Regular License is OK when you install the plugin / theme and use it for yourself or your client.

Any license from above (Regular / Extended) can be used only in one project. The fundamental difference is the way it allows usage.