Currently the licence allows you to use the icons in as many prototypes as you want unless you won't release them. Once you buy it you can play around with them in as many projects as you want.
But The licence also states that it gets bound to a product in case of a crowdfunding campaign or a game release, so if you intend to release a game with the assets, for another project you want to release you need to buy another licence.
This might change in future, however at the moment my goal is to build up a solid chain of products and I need development time to do so. The money I make with the assetstore gets reinvested from my side in updates and new products.