The credits are used to count the email spam tests you create in your account. One test involves sending the message to the GlockApps seed list and getting a deliverability report. Whether you are sending the message to the full or partial seed list, it is still counted as one test and consumes one credit out of the balance.
Automatic tests and tests via proxy addresses count against the total number of tests per month or year. Every time an automatic test works and/or every time you send the message to the proxy address, one credit is deducted from your account.
Unused credits in subscription plans don’t roll over the next month.