1. Pertaining to the art of conducting international relations, as in negotiating alliances, treaties, and agreements: The diplomatic agent traveled to different countries and was successful in maintaining good rapport with other nations.
2. A reference to being tactful and skillful when dealing with people: While growing up, Jill learned that being diplomatic with her parents was quite effective in getting what she wanted, instead of her yelling and behaving badly!

Diplomatic applies to social behavior which is as sensitive as an ambassador's and so it applies to having social behavior that is smooth and courteous.