Even I am not clear but i will share what i know
Lets say you are creating an wp site and you bought a theme from themeforest
if its a regular license you can use it in your website. / client website. but you can’t charge for it.
also its required to get multiple License if incase you want to use it in multiple websites. (Most ppl don’t do that. )
Lets say you are working on a custom application and for that you need a theme. but you are going the app which you developed in that case you need Extended License
Extended License is nothing but you can charge your customers for the app / plugin / assets you bought from Envato to. also you can use on multiple websites i think >!!