Authentication information for the user on whose behalf the application is making the request. Only registered eBay users are allowed to make API calls. To verify that a user is registered, your application needs to pass a user-specific value called an "authentication token" in the request. This is equivalent to signing in on the eBay Web site. As API calls do not pass session information, you need to pass the user's authentication token every time you invoke a call on their behalf. All calls require an authentication token, except the calls you use to retrieve a token in the first place. (For such calls, you use the eBay member's username and password instead.)