CustomerAccountData(AggCat) API and QB REST APIs are two different service.
Ref - https://developer.intuit.com/docs
AggCat - Private Key, Certificate , SAML all these are related to AggCat. If you want an AggCat integration, then refer the following link.
Devkit & Sample apps - https://developer.intuit.com/docs/0020_customeraccountdata/devkits
Docs - https://developer.intuit.com/docs/0020_customeraccountdata
For Aggcat integraion, you should test all keys before integrating those with your application. It will help you to bring the application in running state quickly. To test these keys, you can use apiexploler tool as mentioned in the following docs.
QB - if you interested in integration with QB, then please refer
Devkit & Sample apps - https://developer.intuit.com/docs/0025_quickbooksapi/0055_devkits Docs - https://developer.intuit.com/docs/0025_quickbooksapi
Thanks