In this sample, native client calls a web API and then the web API calls another downstream web API after obtaining a token to act On Behalf Of the original user.  The sample uses the Active Directory Authentication Library (ADAL) in the native client to obtain a token for the user to call the first web API, and also in the first web API to get a token to act on behalf of the user to call the second web API.  Both flows use the OAuth 2.0 protocol to obtain the tokens.