As mentioned earlier, every environment is completely isolated. So any configuration changes made within one environment will not affect the other. This isolation will allow you to experiment and test without any impact on your live environment and live users. Whether it is the general settings, the promo codes, the shipping carriers or any other configuration of the application, they are all isolated within their respective environment.
For instance, all the promo codes you create within your Test environment will only be available when using your Test API Key. They are completely isolated from the Live environment, so you don't have to worry about creating test promo codes: your Live clients will not be able to use them.