I have a basic question. If I create an application, how can I implement that api. Can I pass that api to client to evaluate who does not have ringcentral account?
When you create an Application in developer account here https://developers.ringcentral.com/login.html#/ , the application will expose some APIs based on it's functionality or permission.
For example, if your application is designed to send fax, there is the RingCentral API exposed by this application to test the application passing the application credentials and values. (You can find those in developer portal dashboard of the application)
You can allow you client to use that API to evaluate by providing him the credentials and API arguments.
Also, you can implement that API into any of your own application by integrating it.