With reference to the Friend Smash tutorial as https://developers.facebook.com/docs/games/unity/unity-tutorial what is the relevance of the challenge_brag parameter in the FB.Feed call and the challenge_score parameter in the FB.AppRequest call? As far as I can see they are not explained.
They are just custom parameters that go back to the game. It allows you to pass data through a request so you can handle it when the request is accepted and the invitee goes into the game. You can then call FB.GetDeepLink() to retrieve these params.