Answer:
When you get to a high enough level, you'll get a message from Fleet Command that the V.I. at their training station has "gone rogue" and they want you to go and shut down the core. This opens up the Sol system(our home solar system) in the Local Cluster. The mission is a Mako mission on the Moon(Luna as it's labeled in the game). After you've shut down the core in all three facilities you'll be given the option to select from two different Specialization Classes depending on your base class(6 total). Any points you've already applied to your class skill(i.e. Soldier, Adept, etc...) will be transferred over to your Specialization skill tree.