If your making a call to Hello World in sandbox then you do not have to code to any of the three fields. If you are making an API call then you only need to code in the Request field
For example, when making an API call you enter code in the Request code then you click on the "Run" button. The Response field will show information related to your API call in both the Response field and Response Header field. The Response Header field allows the server to pass additional information about the response.
Hope that helps!
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