Overworld uses a modified version of the Gumshoe rules (especially as laid out by Swords of the Serpentine), and Fate Accelerated (Borrowing heavily from Transhumanity's Fate).
1. Overworld uses the concept of a dual world. They will have an EGO that encompasses their person, and two or more morphs - one on Overword and one in the Consortium. Thus some abilities will not apply when crossing the boundaries between Overworld and the Consortium.
2. While in Overworld, there are properties that will set the PCs apart from others they encounter there. These properties will give them advantages in Overworld, and will also set them apart as different.
3. While in the Consortium, the characters are competent- overly so- in their field. However, the characters are no longer as head-and-shoulders above the competition as they are in Overworld. For the survivability of the PC, one must keep this fact in mind.
For more information on the game, please see the encyclopedia, specifically, the rules section and the character creation section. Also, please peruse the background of the game for more information on this dual-world the characters inhabit. The complete text of the rules are downloadable here.