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 a subscription and purchased credit packs, 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.