Special Forces is a new mode where your cumulative top scores for all missions on a particular difficulty are added together to make up your overall score. This means that replaying missions is encouraged, as you want to get the most points per mission to ensure you score high on the leaderboard. All missions use blue Campaign energy to access. Missions have different requirements in order to qualify for them, such as a specific level range, or a specific weapon type.
For the Alliance leaderboard, your best overall score on any difficulty is added to your Alliance's score, provided you earned that score in the current Alliance. Example: if you got 10,000 on Easy, 20,000 on Medium, and 30,000 on Hard, your score of 30,000 would be added to the Alliance's total, as it's the highest.
If you leave an Alliance, you leave your top score behind with that Alliance. The only way to log a score with a new Alliance is to reach a new high score, higher than the one you left behind.
Note that only the top 25 MVP's in each Alliance will qualify for the Alliance leaderboard reward!