Store source refers to product sources, which you have decided to publish on your stores at virtual marketplaces or e-commerce sites.
You don't have to use every product source that you have identified to the system in every store of yours. You can choose in which stores you are going to use your product sources on store/API sources section. For example if you have identified 4 xml then you have 4 product source. Assume that you have 2 stores, while you are using one of your product source in one store, you can use the other four sources on the other stores. Also, you can make advertisement template settings separately for 2 shops’ xml that you use together and make the sales with different commisions.