Well, I did want to cover most of that in chargen anyway. But here's the campaign-starting "why are you in jail?" question....
You all start on a mid-sized passenger starship, outbound from
Peregrine system, with the first stop being
Jinx. See the wiki for more info on the background if you want to think about it ahead of time.
"Why are you on that starship, leaving Peregrine?" Are you on the run from the local law? Are you on a mission at the behest of an employer/patron? Are you a drifter who was just passing through?
Of course, something important is gonna happen on that trip... your character just doesn't know it yet.
As for available options, it can be any science fiction character within limits... No Jedi or Shapeshifters for example, but you can be an kind of military, colonist, asteroid miner, hitchhiker, star princess, scoundrel or scurvy space pirate... I don't want to type out all of the book's options right now, but we can try to fit most any science fiction concept into the past career archetypes they lay out:
Agent: (Law, Intelligence, Corporate)
Army (Support, Infantry, Cavalry)
Citizen (Corporate, Worker, Colonist)
Drifter (Barbarian, Wanderer, Scavenger)
Entertainer (Artist, Journalist, Performer)
Marines (Support, Star Marine, Ground Assault)
Merchants (Merchant Marine, Free Trader, Broker)
Navy (Line/Crew, Engineering/Gunnery, Flight)
Nobility (Administrator, Diplomat, Dilettante)
Rogue (Thief, Enforcer, Pirate)
Scholar (Field Researcher, Scientist, Physician)
Scout (Courier, Survey, Exploration)
Traveller is a game where you generate your character's stats and then background, which gives you stat bonuses, related skills and skill bonuses, and starting money/ equipment.
Worlds range in tech level, from stone age to high tech... but just because a world is stone age in its ability to produce/work technology, doesn't mean that the ruling class can't have imported laser rifles for the dictator's death squad. As long as the world is on a trade route, some measure of higher technology can be available. You just might not be able to pick up Plutonium-238 at the corner drugstore or get repairs made to your ship's Jump-2 drive locally.
I hope this helps some.
Chris