There are many free documents on the Animaplates website, and there are also "premium" documents that are available at a small cost.
If you have a Basic or Pro Membership, you can pay for the premium documents with credits. Basic Membership includes 20 credits, and Pro Membership includes 40 credits. When you purchase a membership and you use credits, you get a better deal than if you were to buy the documents individually.
Every time you renew your Basic or Pro Membership, you get 20 or 40 credits that are added to your balance of the previous year. As long as you renew your membership every year, you never loose any credits.
If you exhaust all your credits, you can buy more. See the Renew/Upgrade page.