Federal Service comes in two forms, civilian and military and is completely and totally voluntary. You can quit at any time even right before a mission, the only problem with quitting is that you loose out on the benifits of Service. The greatest of these benifits is franchise, the ability to vote. Second to that is the ability to hold Federal office. Though there are a few exceptions to holding Federal Office, they are very few.

Not serving does not deny you the benifits of health care, or other social services, though it does deny you the right to vote. With the restructureing of the military, it has all become one large military, though many still refer to this, or that as Army or Navy, when you sign up, you aren't signing up for this or that branch of service, you sign up for this or that job. Even the Civilian jobs. The process of enrollment is the same.

You must be 18 years of age to enroll in service. Service is the best way to ensure citizenship. You must be a citizen to vote.

The following is a partial listing of possible positions, this is by no means all, and is also subject to change:

Pilot, Transport
Pilot, Fighter
Ground Transport
Intelligence
Logistics and Support
Quartermaster (Civilian)
Special Forces (Gifted)
Special Forces (Non-Gifted)
Space exploration
Research & Development
Medical (Millitary)
Medical (Civilian)
Infantry
Submarine
Ballistics & Support
K-9 Corps

It should be noted that those jobs usually covered by the Air Force have been merged with the Navy, and that the Navy is now responsible for transportation as well as long range bombardments, air support and sea support. The Navy is also responsible for the space transport and space combat. The Marines have merged with the Army and are now more focused on the fighting. There are other groups that deal with support for the infantry. A single infantry unit, cares for itself as best as possible and every member of the unit fights.