When onboarding, you will be provided with two Partner accounts: a sandbox account, and a live account. These will each have their own unique credentials.
The sandbox account should be used during the integration to test out the Partner API, create as many test businesses as you need, and perform test payments. You will not be able to initiate live payments using your sandbox credentials.
You can start using the live account when you have completed the integration. This account will allow you to create live businesses, and to set up live or test business applications (and so to create live or test payments).