Credits purchased in the Pay-as-you-go plan never expire. Even if you have purchased credits and decide to upgrade to a subscription plan, the credits you’ve bought are kept in your account. In other words: if you cancel your subscription, any purchased credits remain at your disposal.
Within a subscription plan, you get a fixed number of credits (50/month in a monthly plan, 600/year in a yearly plan). This number of available subscription credits is always reset when your subscription period renews.
If you both have subscription credits as well as purchased credits, your subscription credits will always be used first for your exports. Only if you’d run out of subscription credits, your purchased credits will be used. In case of high-volume export requirements, you are always able to purchase top-up credits.