Players have formal relationships that may change over the course of the game.
This differs from the Negotiation mechanism in that these relationships are governed by specific game rules. For example, in Dune, players may form alliances at each Nexus phase, which last until the next Nexus. During that time the players win together, may not attack each other, and grant their ally a special power. In Struggle of Empires players bid for the right to select their ally for that round, which is governed by similar rules.
This also differs from Team-Based games in that Alliances may potentially change during the game. If players must be allied for the entire game, and win or lose together, it is a Team-Based Game.