Designate one basic skill as your profession. This is normally one of the main skills (Combat, Engineering, Piloting or Science). You may reroll up to one die in each Skill Check you attempt when using that skill. The skills are color coded to match the modules that use those skills.
Choosing a profession dictates only the skill checks you'll get a reroll in. It does not pigeon-hole you into a role on the ship. Every character will find themselves taking actions outside their profession.
A pilot who says “I just fly the ship” might find his precious Helm occupied by boarders that require some convincing to leave in the form of some Combat. Or the Helm is broken and it takes an Engineering check to fix it. The Scientist is in charge of healing with a MedKit but what if he's knocked out and the unskilled Marine needs to pick up the MedKit to heal him?