The credits show the availability checks count:
let me explain: we use 3 different free WHOIS APIs and one paid WHOIS API, which is used in some cases, when the domain can't be evaluated or like with .org, where the org Whois only allows 5 free queries per day. There are some other WHOIS APIs that throttle the incoming calls. Therefore we implemented a paid-Whois-API with 1000 credits. When the credits are gone, just ask me to renew them.
So basically you have a powerful basic tool for checking domain metrics AND MORE that comes free, where others take money or where many different other tools are needed to achieve the same, like the "text cleaning tool" with the output of domain names while your input is a "trash" text of any characters.
Hope it helps?
Enjoy!
Dan